Widget system Phase 4a: widget CRUD + grid placement UI

Adds the actual "Android home screen" placement experience: a new
Layout tab with a pointer-driven canvas for dragging/resizing widgets
and adding new ones from a type picker. Backed by a new
routers/api_widgets.py (create/move/delete), which re-validates
bounds, minimum footprint, and no-overlap server-side regardless of
what the client already checked. A widget added without an explicit
position lands in the first open space that fits it
(grid.find_open_rect), so users don't have to hunt for empty space
themselves.

Also fixes a real latent bug this surfaced: changing a frame's
orientation swaps the widget grid's long/short axis, which left
existing widget placements out of bounds on the new grid with no
render-time safeguard. Orientation changes now reset the layout to a
single full-panel widget (keeping the first widget's type, dropping
the rest), with a client-side confirm before it happens.

server/app/routers/frame_pages.py

@@ -60,6 +60,11 @@ def frame_photos_page(frame_id: int, request: Request, db: Session = Depends(get
     )
 
 
+@router.get("/frames/{frame_id}/layout", response_class=HTMLResponse)
+def frame_layout_page(frame_id: int, request: Request, db: Session = Depends(get_db)):
+    return _frame_page(request, db, frame_id, "frame_layout.html", "layout")
+
+
 def _user_available_calendars(user: User) -> list[dict]:
     """This user's full set of calendars available to add to any frame:
     the single ICS subscription (if set) plus every CalDAV calendar last