Add Layouts module with Konva.js builder; smart offline polling; UI improvements

- Move board cards from dashboard to top of boards list page
- Fix Werkzeug duplicate polling (WERKZEUG_RUN_MAIN guard)
- Smart offline polling: fast loop for online boards, slow recheck for offline
- Add manual ping endpoint POST /api/boards/<id>/ping
- Add spin animation CSS for ping button

Layouts module (new):
- app/models/layout.py: Layout model (canvas_json, thumbnail_b64)
- app/routes/layouts.py: 5 routes (list, create, builder, save, delete)
- app/templates/layouts/: list and builder templates
- app/static/js/layout_builder.js: full Konva.js builder engine
- app/static/vendor/konva/: vendored Konva.js 9
- Structure mode: wall, room, door, window, fence, text shapes
- Devices mode: drag relay/input/Sonoff channels onto canvas
- Live view mode: click relays/Sonoff to toggle, socket.io state updates
- Device selection: click to select, remove individual device, Delete key
- Fix door/Arc size persistence across save/reload (outerRadius, scaleX/Y)
- Fix Sonoff devices missing from palette (add makeSonoffChip function)

app/static/css/style.css

@@ -51,6 +51,15 @@
   box-shadow: 0 4px 24px rgba(0, 0, 0, 0.4);
 }
 
+/* ── Spinner for ping button ────────────────────────────────────────────── */
+@keyframes spin {
+  to { transform: rotate(360deg); }
+}
+.spin {
+  display: inline-block;
+  animation: spin 0.8s linear infinite;
+}
+
 /* ── Relay buttons ──────────────────────────────────────────────────────── */
 .relay-btn {
   min-width: 90px;