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/ja/common.json

@@ -686,6 +686,38 @@
     "recipient_name_placeholder": "表示名",
     "autocomplete_search_server": "サーバーを検索",
     "autocomplete_searching": "検索中...",
+    "toolbar": {
+      "bold": "太字",
+      "italic": "斜体",
+      "underline": "下線",
+      "strikethrough": "取り消し線",
+      "text_color": "文字色",
+      "remove_color": "色を解除",
+      "heading_1": "見出し1",
+      "heading_2": "見出し2",
+      "bullet_list": "箇条書き",
+      "ordered_list": "番号付きリスト",
+      "quote": "引用",
+      "code_block": "コードブロック",
+      "align_left": "左揃え",
+      "align_center": "中央揃え",
+      "align_right": "右揃え",
+      "text_direction": "文字方向 (RTL/LTR)",
+      "link": "リンク",
+      "table": "表",
+      "clear_formatting": "書式をクリア",
+      "undo": "元に戻す",
+      "redo": "やり直す",
+      "add_row_above": "上に行を追加",
+      "add_row_below": "下に行を追加",
+      "add_column_before": "左に列を追加",
+      "add_column_after": "右に列を追加",
+      "delete_row": "行を削除",
+      "delete_column": "列を削除",
+      "toggle_header_row": "ヘッダー行の切り替え",
+      "delete_table": "表を削除",
+      "pick_size": "サイズを選択"
+    },
     "send_filing_warning": "送信されましたが、送信後の整理に失敗しました。古い下書きが残る場合があります。"
   },
   "confirm_dialog": {