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/de/common.json

@@ -1482,6 +1482,9 @@
     "title": "Kontakte",
     "search_placeholder": "Kontakte suchen...",
     "create_new": "Neuer Kontakt",
+    "no_category": "Ohne Kategorie",
+    "category_added": "Kontakt zu {name} hinzugefügt",
+    "category_added_plural": "{count} Kontakte zu {name} hinzugefügt",
     "empty_state": "Keine Kontakte",
     "empty_state_title": "Keine Kontakte",
     "empty_state_subtitle": "Erstellen Sie Ihren ersten Kontakt oder importieren Sie aus einer vCard-Datei",
@@ -1625,7 +1628,8 @@
       "level_low": "Niedrig",
       "categories": "Kategorien",
       "categories_placeholder": "z. B. Familie, Freunde, Kollegen",
-      "categories_hint": "Mit Kommas trennen",
+      "categories_hint": "Tippen zum Suchen oder Hinzufügen",
+      "category_add": "Hinzufügen",
       "note": "Notizen",
       "note_placeholder": "Notiz hinzufügen...",
       "gender": "Geschlecht",