Redesign doc service UX for scale + add group-based document sharing

- Three-column layout: Sidebar + SourcePanel (views + searchable category tree) + main
- DocumentSlideOver (480px right panel): inline editing, type picker, AI suggestion confirm/reject,
  categories combobox, tags editor, sharing section, raw text, re-analyse/delete actions
- ManageCategoriesDialog: inline rename, delete with confirm, search filter
- DocumentsPage rewrite: filter chip system, multi-file upload queue, drag-and-drop overlay,
  bulk actions bar (share/delete), smart TanStack Query polling, URL-driven view state
- Sidebar simplified: per-category NavLinks removed; Documents = single NavLink under Apps
- Backend: document_shares table (migration 0004), share CRUD endpoints, shared-with-me view,
  N+1-safe share_count via GROUP BY, recipient download access, X-User-Groups header enforcement
- Gateway proxy: injects X-User-Groups header into all document + category proxy requests
- Backend users: GET /api/users/me/groups endpoint for share picker combobox
- CLAUDE.md, STATUS.md files, and changelog updated

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

backend/app/routers/users.py

@@ -1,10 +1,13 @@
 from fastapi import APIRouter, Depends, HTTPException
+from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
+from sqlalchemy.orm import selectinload
 
 from app.database import get_db
 from app.deps import get_current_user
+from app.models.group import Group, GroupMembership
 from app.models.user import User
-from app.schemas.user import ColorModeUpdate, DashboardPrefsOut, DashboardPrefsUpdate, UserOut
+from app.schemas.user import ColorModeUpdate, DashboardPrefsOut, DashboardPrefsUpdate, UserGroupOut, UserOut
 
 router = APIRouter()
 
@@ -31,6 +34,21 @@ async def update_preferences(
     return DashboardPrefsOut(app_ids=current_user.dashboard_app_ids or [])
 
 
+@router.get("/me/groups", response_model=list[UserGroupOut])
+async def get_my_groups(
+    current_user: User = Depends(get_current_user),
+    db: AsyncSession = Depends(get_db),
+):
+    """Return all groups the current user belongs to."""
+    result = await db.execute(
+        select(Group)
+        .join(GroupMembership, GroupMembership.group_id == Group.id)
+        .where(GroupMembership.user_id == current_user.id)
+        .order_by(Group.name)
+    )
+    return result.scalars().all()
+
+
 @router.patch("/me/color-mode", response_model=UserOut)
 async def update_color_mode(
     body: ColorModeUpdate,