fix(i18n): add missing translation keys for signatures and email import

Fixes 25 failing tests across translations and eml-import test suites:

- Add 141 missing i18n keys to en/common.json for P2.1 (extended signatures
  feature) and P2.5 (email import feature). Keys cover:
  - signatures.* (signature editor, toolbar labels, per-identity settings)
  - settings.importer.* (mail import UI labels and messages)
  - settings.tabs.signatures, settings.{loading,refresh}
  - email_composer.{insert_signature,no_signature,select_signature}
  - identities.form.{signature_store_*,use_global_default}
  - calendar.{edit,delete,duplicate}, contacts.{edit,delete,send_email}
  - contacts.import.csv_* (CSV column mapping labels)
  - Other related keys for sharing, mailbox context menu, file operations

- Propagate new keys to all 23 other locale files:
  - ar, ca, cs, da, de, es, fa, fr, he, hu, it, ja, ko, lv, nl, pl, pt, ro,
    ru, sk, tr, uk, zh
  - 19 locales with real, idiomatic translations matching existing tone
  - 4 locales (da, es, fa, zh) with English placeholders pending native
    translation

- Fix eml-import test: update accept-string expectation to match actual
  EML_IMPORT_ACCEPT constant (now includes .tgz, .tar.gz, application/gzip
  support added in P2.5)

All 53 tests now pass:
  - 48 translation completeness tests
  - 5 eml-import tests

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

locales/zh/common.json

@@ -567,7 +567,8 @@
       "copied": "已复制！",
       "copy_failed": "复制失败"
     },
-    "send_now": "立即发送"
+    "send_now": "立即发送",
+    "create_appointment": "Create Appointment"
   },
   "email_composer": {
     "read_receipt_on": "已请求已读回执（点击以关闭）",
@@ -712,7 +713,10 @@
       "delete_table": "删除表格",
       "pick_size": "选择大小"
     },
-    "send_filing_warning": "已发送，但发送后的清理失败，可能会残留旧草稿。"
+    "send_filing_warning": "已发送，但发送后的清理失败，可能会残留旧草稿。",
+    "insert_signature": "Insert signature",
+    "no_signature": "No signature",
+    "select_signature": "Select signature"
   },
   "confirm_dialog": {
     "confirm": "确认",
@@ -888,7 +892,10 @@
       "downloads": "下载",
       "content_senders": "内容和发件人",
       "about_data": "关于和数据",
-      "debug": "调试"
+      "debug": "调试",
+      "import": "Import",
+      "sharing": "Sharing",
+      "signatures": "Signatures"
     },
     "tab_groups": {
       "general": "通用",
@@ -2007,7 +2014,39 @@
     "scoped": {
       "back": "返回我的账户",
       "managing": "管理：{name}"
-    }
+    },
+    "importer": {
+      "action_label": "Import",
+      "cancel": "Cancel",
+      "choose_files": "Choose files",
+      "conflict_copy": "Keep both",
+      "conflict_description": "Choose what to do when an imported message already exists.",
+      "conflict_label": "Duplicate handling",
+      "conflict_replace": "Replace duplicates",
+      "conflict_skip": "Skip duplicates",
+      "description": "Import email messages from .eml files into a folder.",
+      "error_details": "{count, plural, one {# error} other {# errors}}",
+      "fail": "Import failed",
+      "file_description": "Select one or more .eml files to import.",
+      "file_label": "Files",
+      "files_selected": "{count, plural, one {# file selected} other {# files selected}}",
+      "folder_description": "Choose the folder to import messages into.",
+      "folder_label": "Destination folder",
+      "import_complete": "Import complete",
+      "import_more": "Import more",
+      "importing": "Importing...",
+      "progress_failed": "{count} failed",
+      "progress_imported": "{count} imported",
+      "progress_skipped": "{count} skipped",
+      "start_import": "{count, plural, one {Import # file} other {Import # files}}",
+      "success": "{count, plural, one {# message imported} other {# messages imported}}",
+      "summary_failed": "{count, plural, one {# message failed} other {# messages failed}}",
+      "summary_imported": "{count, plural, one {# message imported} other {# messages imported}}",
+      "summary_skipped": "{count, plural, one {# message skipped} other {# messages skipped}}",
+      "title": "Import Mail"
+    },
+    "loading": "Loading...",
+    "refresh": "Refresh"
   },
   "errors": {
     "page_error_title": "出了点问题",
@@ -2085,7 +2124,8 @@
     "toast_error_delete_has_email": "文件夹不为空，请先清空它。",
     "placeholder_folder_name": "文件夹名称",
     "create": "创建",
-    "rename_confirm": "重命名"
+    "rename_confirm": "重命名",
+    "share_folder": "Share Folder..."
   },
   "shortcuts": {
     "title": "键盘快捷键",
@@ -2184,7 +2224,11 @@
       "save": "保存身份",
       "cancel": "取消",
       "creating": "创建中...",
-      "updating": "更新中..."
+      "updating": "更新中...",
+      "signature_store_default": "Default signature",
+      "signature_store_mapping": "Signature mapping",
+      "signature_store_reply": "Reply signature",
+      "use_global_default": "Use global default"
     },
     "sub_address": {
       "button_tooltip": "使用子地址",
@@ -2510,7 +2554,29 @@
       "success": "{count, plural, one {已导入 1 位联系人} other {已导入 # 位联系人}}",
       "failed": "导入失败",
       "close": "关闭",
-      "file_too_large": "文件太大（最大 5 MB）"
+      "file_too_large": "文件太大（最大 5 MB）",
+      "csv_address": "Address",
+      "csv_address_book": "Address book",
+      "csv_back": "Back",
+      "csv_city": "City",
+      "csv_company": "Company",
+      "csv_country": "Country",
+      "csv_email": "Email",
+      "csv_first_name": "First name",
+      "csv_ignore": "Ignore this column",
+      "csv_job_title": "Job title",
+      "csv_last_name": "Last name",
+      "csv_load_all": "Load all",
+      "csv_map_columns": "Map columns",
+      "csv_nickname": "Nickname",
+      "csv_note": "Note",
+      "csv_phone": "Phone",
+      "csv_postcode": "Postal code",
+      "csv_preview": "Preview",
+      "csv_preview_title": "Preview ({count, plural, one {# row} other {# rows}})",
+      "csv_region": "State/Region",
+      "csv_website": "Website",
+      "file_types_csv": ".csv files"
     },
     "export": {
       "title": "导出联系人",
@@ -2569,7 +2635,10 @@
       "has_phone": "有电话",
       "has_photo": "有照片"
     },
-    "open_categories": "打开分类"
+    "open_categories": "打开分类",
+    "delete": "Delete",
+    "edit": "Edit",
+    "send_email": "Send email"
   },
   "calendar": {
     "title": "日历",
@@ -2988,7 +3057,37 @@
       "bah": "Bahman",
       "esf": "Esfand"
     },
-    "nav_open_menu": "打开菜单"
+    "nav_open_menu": "打开菜单",
+    "delete": "Delete",
+    "duplicate": "Duplicate",
+    "edit": "Edit",
+    "freeBusy": {
+      "busy": "Busy",
+      "check": "Check Availability",
+      "click_to_select": "Click a free slot to select this time",
+      "free": "Free",
+      "hide": "Hide Availability",
+      "loading": "Loading...",
+      "no_participants": "Add participants to check availability.",
+      "tentative": "Tentative",
+      "timezone": "Timezone",
+      "title": "Availability",
+      "unavailable": "Out of office",
+      "unknown": "No information"
+    },
+    "resources": {
+      "clear_all": "Clear all",
+      "filter_all": "All",
+      "hide": "Hide resources",
+      "no_resources": "No resources available",
+      "remove": "Remove {name}",
+      "search_placeholder": "Search resources...",
+      "title": "Resources",
+      "type_equipment": "Equipment",
+      "type_other": "Other",
+      "type_room": "Rooms",
+      "type_vehicle": "Vehicles"
+    }
   },
   "advanced_search": {
     "title": "高级搜索",
@@ -3153,7 +3252,8 @@
     "open_folder_tree": "打开文件夹树",
     "other_accounts": "其他账户",
     "migration_title": "正在更新您的文件…",
-    "migration_description": "正在将文件夹和文件整理为正确的结构。此操作仅执行一次。"
+    "migration_description": "正在将文件夹和文件整理为正确的结构。此操作仅执行一次。",
+    "send_as_attachment": "Send as Attachment"
   },
   "smime": {
     "your_certificates": "您的证书",
@@ -3308,7 +3408,14 @@
       "readWrite": "读写",
       "manager": "管理员",
       "custom": "自定义"
-    }
+    },
+    "accept": "Accept",
+    "decline": "Decline",
+    "no_shares_by_me": "You haven't shared anything yet.",
+    "no_shares_with_me": "No folders shared with you yet.",
+    "shared_by": "Shared by",
+    "tab_shared_by_me": "Shared by me",
+    "tab_shared_with_me": "Shared with me"
   },
   "quote_header": {
     "reply_line": "在 {date}，{from} 写道：",
@@ -3324,5 +3431,55 @@
     "install": "安装",
     "dont_remind": "不再提醒",
     "dismiss_aria": "关闭安装提示"
+  },
+  "signatures": {
+    "add_signature": "Add signature",
+    "default": "Default",
+    "default_signature": {
+      "description": "Used for new messages unless overridden per identity.",
+      "label": "Default signature"
+    },
+    "delete_message": "Are you sure you want to delete \"{name}\"? This cannot be undone.",
+    "delete_title": "Delete signature?",
+    "description": "Create and manage email signatures to use when composing or replying.",
+    "duplicate": "Duplicate",
+    "edit_signature": "Edit signature",
+    "editor_label": "Signature",
+    "html_preview_label": "HTML preview",
+    "name_label": "Name",
+    "name_placeholder": "e.g., Work, Personal",
+    "name_required": "Name is required",
+    "new_signature": "New signature",
+    "no_signature": "No signature",
+    "no_signatures": "No signatures yet",
+    "per_identity_signatures": {
+      "description": "Override the default and reply signature for individual identities.",
+      "label": "Per-identity signatures"
+    },
+    "plain_text_preview_label": "Plain text preview",
+    "reply": "Reply",
+    "reply_signature": {
+      "description": "Used when replying or forwarding unless overridden per identity.",
+      "label": "Reply signature"
+    },
+    "show_editor": "Show editor",
+    "show_preview": "Show preview",
+    "title": "Signatures",
+    "toolbar": {
+      "align_center": "Align center",
+      "align_left": "Align left",
+      "align_right": "Align right",
+      "bold": "Bold",
+      "bullet_list": "Bullet list",
+      "italic": "Italic",
+      "link": "Link",
+      "ordered_list": "Ordered list",
+      "remove_color": "Remove color",
+      "strikethrough": "Strikethrough",
+      "text_color": "Text color",
+      "underline": "Underline"
+    },
+    "use_global_default": "Use global default",
+    "your_signatures": "Your signatures ({count})"
   }
 }