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/nl/common.json

@@ -1482,6 +1482,9 @@
     "title": "Contacten",
     "search_placeholder": "Contacten zoeken...",
     "create_new": "Nieuw contact",
+    "no_category": "Geen categorie",
+    "category_added": "Contact toegevoegd aan {name}",
+    "category_added_plural": "{count} contacten toegevoegd aan {name}",
     "empty_state": "Geen contacten",
     "empty_state_title": "Geen contacten",
     "empty_state_subtitle": "Maak uw eerste contact aan of importeer vanuit een vCard-bestand",
@@ -1625,7 +1628,8 @@
       "level_low": "Laag",
       "categories": "Categorieën",
       "categories_placeholder": "bijv. Familie, Vrienden, Collega's",
-      "categories_hint": "Scheiden met komma's",
+      "categories_hint": "Typ om te zoeken of toe te voegen",
+      "category_add": "Toevoegen",
       "note": "Notities",
       "note_placeholder": "Notitie toevoegen...",
       "gender": "Geslacht",