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/static/frame_config.js

@@ -27,10 +27,31 @@ async function saveConfig() {
   }
 }
 
+// Orientation swaps the widget grid's long/short axis (see
+// grid.grid_dims), so an existing widget layout is usually left with
+// out-of-bounds coordinates on the new grid -- the server resets it to
+// one full-panel widget when this actually changes (see
+// api_frames.py's api_config_save). Warn before that happens rather
+// than silently losing whatever layout was on the Layout tab.
+let lastSavedOrientation = document.getElementById('orientation').value;
+
 document.getElementById('config-form').addEventListener('submit', async (e) => {
   e.preventDefault();
+  const newOrientation = document.getElementById('orientation').value;
+  if (newOrientation !== lastSavedOrientation) {
+    const proceed = confirm(
+      "Changing orientation resets this frame's widget layout to a single " +
+      'full-panel widget -- any other widgets placed on the Layout tab will ' +
+      'be removed. Continue?'
+    );
+    if (!proceed) {
+      document.getElementById('orientation').value = lastSavedOrientation;
+      return;
+    }
+  }
   try {
     await saveConfig();
+    lastSavedOrientation = newOrientation;
     showStatus(true, 'Saved.');
   } catch (e) {
     showStatus(false, e.message);