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/theme.css

@@ -167,6 +167,8 @@ h2.card-title, summary.card-title {
 summary.card-title { cursor: pointer; margin-bottom: 0; }
 details.card[open] summary.card-title { margin-bottom: 14px; }
 details.card .sub { margin-top: 8px; }
+.card-title-row { display: flex; align-items: center; justify-content: space-between; gap: 10px; margin-bottom: 14px; }
+.card-title-row .card-title { margin-bottom: 0; }
 
 .palette-table-wrap { overflow-x: auto; margin-top: 14px; }
 .palette-table { width: 100%; border-collapse: collapse; font-size: 13px; }
@@ -312,6 +314,7 @@ button {
 }
 button:hover { background: var(--accent-hover); }
 button:active { transform: translateY(1px); }
+button:disabled { opacity: 0.5; cursor: default; pointer-events: none; }
 button.btn-inline {
   margin-top: 0;
   padding: 3px 10px;
@@ -514,7 +517,12 @@ code {
 }
 .main-col, .side-col { display: flex; flex-direction: column; }
 @media (max-width: 860px) {
-  .layout { grid-template-columns: 1fr; }
+  /* minmax(0, 1fr), not bare 1fr -- bare 1fr is minmax(auto, 1fr), whose
+     "auto" minimum lets the track (and everything in it) grow to fit its
+     widest descendant's min-content size instead of actually shrinking
+     to the viewport, causing page-wide horizontal overflow/scroll on
+     narrow screens. Same fix the desktop rule above already applies. */
+  .layout { grid-template-columns: minmax(0, 1fr); }
 }
 /* ------------------------------------------------------------------ */
 /* App shell: left sidebar (frame list + account nav) + main content. */