feat: navigation restructure + drag-and-drop menu ordering

Navigation:
- Remove Plugins header from sidebar
- Merge static items and plugin items into unified sorted list
- Group related menu items (Dateien, E-Mail, Kalender, Automation)
- German labels for all menu items
- Sort by order field, alphabetical fallback for ties

Drag-and-Drop Menu:
- New backend endpoints: GET/PUT /api/v1/users/me/menu-order
- Store menu order in user preferences JSONB field
- New SettingsMenuOrder page with @dnd-kit drag-and-drop
- New Settings tab: Menu ordering
- Sidebar reads saved menu order and sorts accordingly
- Reset to default option

app/routes/users.py

@@ -13,7 +13,7 @@ from app.core.auth import get_redis
 from app.core.db import get_db
 from app.core.notifications import create_notification
 from app.core.permissions import invalidate_permission_cache
-from app.deps import require_permission
+from app.deps import get_current_user, require_permission
 from app.schemas.user import UserCreate, UserUpdate, UserResponse, PaginatedUsers
 from app.services.user_service import user_service, _UNSET
 
@@ -276,3 +276,61 @@ async def delete_user(
     )
 
     return Response(status_code=status.HTTP_204_NO_CONTENT)
+
+
+@router.get("/me/menu-order")
+async def get_menu_order(
+    db: AsyncSession = Depends(get_db),
+    current_user: dict = Depends(get_current_user),
+):
+    """Get the current user's menu order preference."""
+    tenant_id = uuid.UUID(current_user["tenant_id"])
+    user_id = uuid.UUID(current_user["user_id"])
+
+    from sqlalchemy import select
+    from app.models.user import User
+
+    result = await db.execute(
+        select(User).where(User.id == user_id, User.tenant_id == tenant_id)
+    )
+    user = result.scalar_one_or_none()
+    if user is None:
+        raise HTTPException(404, detail={"detail": "User not found", "code": "not_found"})
+
+    menu_order = user.preferences.get("menu_order", [])
+    return {"menu_order": menu_order}
+
+
+@router.put("/me/menu-order")
+async def update_menu_order(
+    body: dict,
+    db: AsyncSession = Depends(get_db),
+    current_user: dict = Depends(get_current_user),
+):
+    """Update the current user's menu order preference."""
+    tenant_id = uuid.UUID(current_user["tenant_id"])
+    user_id = uuid.UUID(current_user["user_id"])
+
+    from sqlalchemy import select
+    from app.models.user import User
+
+    menu_order = body.get("menu_order")
+    if not isinstance(menu_order, list) or not all(isinstance(x, str) for x in menu_order):
+        raise HTTPException(
+            400,
+            detail={"detail": "menu_order must be a list of strings", "code": "invalid_format"},
+        )
+
+    result = await db.execute(
+        select(User).where(User.id == user_id, User.tenant_id == tenant_id)
+    )
+    user = result.scalar_one_or_none()
+    if user is None:
+        raise HTTPException(404, detail={"detail": "User not found", "code": "not_found"})
+
+    prefs = dict(user.preferences) if user.preferences else {}
+    prefs["menu_order"] = menu_order
+    user.preferences = prefs
+    await db.commit()
+
+    return {"menu_order": menu_order}