feat: standalone focus items with edit, convert, project tab, domain ordering

- Add standalone text line items to focus (quick-add with optional domain)
- Edit page for standalone items (title, domain, project)
- Convert standalone items to task, note, link, or list item
- Focus tab on project detail page showing assigned focus items
- Sort domain groups: General first, then by domain sort_order
- Add domain_id and title to nullable_fields in BaseRepository

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

core/base_repository.py

@@ -162,6 +162,7 @@ class BaseRepository:
             "contact_id", "started_at",
             "weekly_hours", "effective_from",
             "task_id", "meeting_id", "list_item_id",
+            "domain_id", "title",
         }
         clean_data = {}
         for k, v in data.items():