Mark whiteboard as (alpha); add the Phase 5 button-assignment UI

Whiteboard rendering isn't fully reliable yet -- tag it (alpha)
everywhere it's user-facing (widget label, add-widget button, dialog
title, Settings' WebDAV section) via one shared WIDGET_LABELS map
(moved to common.js so both the Layout canvas and the new Configuration
tab section can use it).

Button assignments: a new "Button assignments" card on the
Configuration tab lets you assign an ordered list of (widget, action)
bindings to each physical NEXT/BACK button -- add/remove/reorder, all
autosaved. Backed by new GET/PUT /api/frames/{id}/buttons endpoints;
PUT replaces a button's whole list in one atomic, fully-validated call
rather than separate add/remove/reorder endpoints. Device-side
consumption already existed (routers/device.py's _run_button_actions);
this is the UI for what was previously only reachable via the default
migration mapping.

server/app/static/frame_layout.js

@@ -18,7 +18,8 @@
 
 let gridState = null; // last-loaded GET .../widgets response
 
-const WIDGET_LABELS = { photos: 'Photos', calendar: 'Calendar', whiteboard: 'Whiteboard' };
+// WIDGET_LABELS comes from common.js (shared with frame_config.js's
+// button-assignment UI).
 
 function renderControlBanner(control) {
   const banner = document.getElementById('control-banner');