i18n: localize the editor toolbar in all 23 locales

The rich-text editor was the last hardcoded-English surface in the
composer: 21 tooltip titles, the eight table-menu entries, the "Remove
color" entry and the table size picker's "Pick size" label were plain
strings while every other menu in the app is localized.

All of them now come from a new email_composer.toolbar namespace,
translated into all 23 locales using each platform's established
editor terminology (Word/Docs conventions - de "Formatierung löschen",
ar "مسح التنسيق", ja "書式をクリア", ...). The link prompt stays "URL",
which is the same term in every language.

Tooltips and self-sizing dropdowns have no width constraints, so longer
translations are safe everywhere.

locales/zh/common.json

@@ -686,6 +686,38 @@
     "recipient_name_placeholder": "显示名称",
     "autocomplete_search_server": "在服务器上搜索",
     "autocomplete_searching": "搜索中...",
+    "toolbar": {
+      "bold": "加粗",
+      "italic": "斜体",
+      "underline": "下划线",
+      "strikethrough": "删除线",
+      "text_color": "文字颜色",
+      "remove_color": "移除颜色",
+      "heading_1": "标题 1",
+      "heading_2": "标题 2",
+      "bullet_list": "项目符号列表",
+      "ordered_list": "编号列表",
+      "quote": "引用",
+      "code_block": "代码块",
+      "align_left": "左对齐",
+      "align_center": "居中对齐",
+      "align_right": "右对齐",
+      "text_direction": "文字方向 (RTL/LTR)",
+      "link": "链接",
+      "table": "表格",
+      "clear_formatting": "清除格式",
+      "undo": "撤销",
+      "redo": "重做",
+      "add_row_above": "在上方添加行",
+      "add_row_below": "在下方添加行",
+      "add_column_before": "在前面添加列",
+      "add_column_after": "在后面添加列",
+      "delete_row": "删除行",
+      "delete_column": "删除列",
+      "toggle_header_row": "切换标题行",
+      "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。"
   },
   "confirm_dialog": {