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/grid.py

@@ -65,6 +65,21 @@ def overlaps(a: Rect, b: Rect) -> bool:
     return ax < bx + bw and bx < ax + aw and ay < by + bh and by < ay + ah
 
 
+def find_open_rect(orientation: str, existing: list[Rect], w: int, h: int) -> Rect | None:
+    """First w x h rect that's in-bounds and doesn't overlap any of
+    `existing`, scanning row-major (top-left first) -- used when creating
+    a widget without an explicit placement (see routers/api_widgets.py),
+    so adding one from a type picker doesn't require the caller to find
+    empty space itself first. None if no such rect fits anywhere."""
+    cols, rows = grid_dims(orientation)
+    for y in range(rows - h + 1):
+        for x in range(cols - w + 1):
+            candidate = (x, y, w, h)
+            if not any(overlaps(candidate, other) for other in existing):
+                return candidate
+    return None
+
+
 def cell_to_pixels(orientation: str, panel_w: int, panel_h: int, rect: Rect) -> tuple[int, int, int, int]:
     """Grid rect -> pixel rect in logical (pre-rotation) canvas space --
     against image_pipeline.logical_render_size(orientation)'s own