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/en/common.json

@@ -1495,6 +1495,9 @@
     "title": "Contacts",
     "search_placeholder": "Search contacts...",
     "create_new": "New Contact",
+    "no_category": "No Category",
+    "category_added": "Contact added to {name}",
+    "category_added_plural": "{count} contacts added to {name}",
     "empty_state": "No contacts yet",
     "empty_state_title": "No contacts yet",
     "empty_state_subtitle": "Create your first contact or import from a vCard file",
@@ -1638,7 +1641,8 @@
       "level_low": "Low",
       "categories": "Categories",
       "categories_placeholder": "e.g., Family, Friends, Colleagues",
-      "categories_hint": "Separate with commas",
+      "categories_hint": "Type to search or add categories",
+      "category_add": "Add",
       "note": "Notes",
       "note_placeholder": "Add a note...",
       "gender": "Gender",