Add a "Clear all" button to the Layout tab's widget canvas

One request (DELETE /api/frames/{id}/widgets) removes every widget on
the frame in a single locked transaction, cascading their configs and
button-action bindings the same way single-widget delete already does.
Gated behind confirm() like the existing per-widget remove button, and
disabled when there's nothing to clear. Covered by the same owner/
unrelated-user/linked-but-not-controlling permission shape used
elsewhere (test_widget_placement.py).

Also fixes a real, pre-existing mobile bug this surfaced: the Layout
page's .layout grid used a bare `1fr` track on the <860px breakpoint
instead of `minmax(0, 1fr)` like the desktop rule already does, so a
wide enough descendant (previously nothing hit this; the new title-row
button did) would force the whole page into horizontal scroll on phone
widths. Verified before/after with the run-server driver's new
`viewport` command.

server/app/static/frame_layout.js

@@ -160,6 +160,23 @@ async function removeWidget(id) {
   }
 }
 
+async function clearAllWidgets() {
+  const count = gridState ? gridState.widgets.length : 0;
+  if (!count) return;
+  const noun = count === 1 ? 'widget' : 'widgets';
+  if (!confirm(`Remove all ${count} ${noun} from this frame and start over? Their settings will be lost.`)) return;
+  try {
+    const resp = await fetch(`${window.FRAME_API}/widgets`, { method: 'DELETE' });
+    if (!resp.ok) throw new Error(await apiError(resp));
+    showStatus(true, 'Cleared.');
+  } catch (e) {
+    showStatus(false, e.message);
+  } finally {
+    loadWidgets();
+  }
+}
+document.getElementById('clear-all-widgets').addEventListener('click', clearAllWidgets);
+
 async function addWidget(widgetType) {
   try {
     const resp = await fetch(`${window.FRAME_API}/widgets`, {
@@ -180,6 +197,7 @@ function renderCanvas() {
   const canvas = document.getElementById('widget-canvas');
   canvas.innerHTML = '';
   document.getElementById('widget-canvas-empty-hint').style.display = gridState.widgets.length ? 'none' : '';
+  document.getElementById('clear-all-widgets').disabled = !gridState.widgets.length;
 
   for (const widget of gridState.widgets) {
     const box = document.createElement('div');