feat(contacts): add no-category filter, drag-drop to category, and category combo box

- Add 'No Category' sidebar item to filter uncategorized contacts
- Categories section now always visible (not just when keywords exist)
- Add drag-and-drop support on category items in sidebar to assign keywords
- Fix effectAllowed mismatch (move -> copyMove) for category drop targets
- Replace plain text categories input with combo box in contact edit form
  - Shows existing categories as clickable suggestions
  - Displays assigned categories as removable badges
  - Supports adding new categories inline
- Add translations for all 8 locales

locales/ja/common.json

@@ -1482,6 +1482,9 @@
     "title": "連絡先",
     "search_placeholder": "連絡先を検索...",
     "create_new": "新しい連絡先",
+    "no_category": "カテゴリなし",
+    "category_added": "{name} に連絡先を追加しました",
+    "category_added_plural": "{count} 件の連絡先を {name} に追加しました",
     "empty_state": "連絡先がありません",
     "empty_state_title": "連絡先がありません",
     "empty_state_subtitle": "最初の連絡先を作成するか、vCardファイルからインポートしてください",
@@ -1625,7 +1628,8 @@
       "level_low": "低",
       "categories": "カテゴリー",
       "categories_placeholder": "例：家族、友人、同僚",
-      "categories_hint": "カンマで区切ってください",
+      "categories_hint": "検索または追加するには入力",
+      "category_add": "追加",
       "note": "メモ",
       "note_placeholder": "メモを追加...",
       "gender": "性別",