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/api_frames.py

@@ -25,7 +25,7 @@ from pydantic import BaseModel
 from sqlalchemy import select
 from sqlalchemy.orm import Session
 
-from .. import calendar_render, gitea_releases, photo_queue, quiet_hours, weather, webdav_client
+from .. import calendar_render, gitea_releases, grid, photo_queue, quiet_hours, weather, webdav_client
 from ..auth import require_frame_control, require_frame_view, require_user_api
 from ..db import frame_locked, get_db, widget_locked
 from ..image_pipeline import (
@@ -36,7 +36,15 @@ from ..image_pipeline import (
     render_preview_png,
 )
 from ..firmware import firmware_path, parse_app_version
-from ..models import BatteryLog, CalendarWidgetConfig, Frame, FrameCalendar, PhotoWidgetConfig, WhiteboardWidgetConfig
+from ..models import (
+    BatteryLog,
+    CalendarWidgetConfig,
+    Frame,
+    FrameCalendar,
+    PhotoWidgetConfig,
+    WhiteboardWidgetConfig,
+    Widget,
+)
 from .common import (
     OVERDUE_FACTOR,
     battery_estimate_s,
@@ -67,6 +75,32 @@ MAX_QUEUE_TARGET_LEN = 5000
 ORIENTATIONS = ("landscape", "portrait", "landscape_flipped", "portrait_flipped")
 
 
+def _reset_widget_layout_for_new_orientation(db: Session, frame_id: int, new_orientation: str) -> None:
+    """A widget's x/y/w/h are grid cells relative to the OLD orientation's
+    cols x rows (see grid.grid_dims) -- landscape and portrait use a
+    transposed grid (8x5 vs 5x8), so an existing placement is often
+    literally out of bounds on the new grid, not just visually wrong.
+    There's no sensible coordinate remap between two differently-shaped
+    grids, so instead: keep whichever widget was first by placement
+    order, resized to fill the new full panel, and delete the rest --
+    cascading to their own config rows and any FrameButtonAction
+    bindings via ondelete="CASCADE" (see models.py). The frontend is
+    expected to confirm this with the user before submitting an
+    orientation change (see frame_config.js) -- this always executes
+    unconditionally once called, same posture as every other
+    confirm-on-the-client / act-unconditionally-on-the-server action in
+    this codebase."""
+    widgets = db.scalars(
+        select(Widget).where(Widget.frame_id == frame_id).order_by(Widget.sort_order)
+    ).all()
+    if not widgets:
+        return
+    keep, *rest = widgets
+    for widget in rest:
+        db.delete(widget)
+    keep.x, keep.y, keep.w, keep.h = grid.full_panel_rect(new_orientation)
+
+
 @router.get("/api/frames/{frame_id}/albums")
 def api_albums(frame: Frame = Depends(require_frame_view)):
     url, key = immich_creds(frame)
@@ -124,6 +158,12 @@ def api_config_save(
     harmless no-ops if an old cached page still POSTs them -- FastAPI
     silently ignores form fields with no matching parameter.
 
+    An actual orientation *change* resets the frame's widget layout (see
+    _reset_widget_layout_for_new_orientation) -- widget placement is
+    grid-cell-relative to the panel's long/short axis, which swaps on a
+    landscape<->portrait change, so an old placement is usually not just
+    visually wrong but literally out of bounds on the new grid.
+
     Until the widget-placement UI (a later phase) lets a frame have more
     than one widget of a type, "the photo widget" / "the calendar
     widget" below unambiguously means the frame's single auto-migrated
@@ -138,7 +178,10 @@ def api_config_save(
                 MIN_REFRESH_INTERVAL_S, min(MAX_REFRESH_INTERVAL_S, refresh_interval_s)
             )
         if orientation is not None:
-            cfg.orientation = orientation if orientation in ORIENTATIONS else "landscape"
+            new_orientation = orientation if orientation in ORIENTATIONS else "landscape"
+            if new_orientation != cfg.orientation:
+                _reset_widget_layout_for_new_orientation(db, cfg.id, new_orientation)
+            cfg.orientation = new_orientation
         if quiet_hours_enabled is not None:
             cfg.quiet_hours_enabled = quiet_hours_enabled
         if quiet_hours_start is not None and quiet_hours.valid_hhmm(quiet_hours_start):