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/common.js

@@ -58,6 +58,10 @@
   }
 })();
 
+// Shared display names for widget_type, everywhere one shows up in the
+// UI (Layout canvas, Add-a-widget buttons, button-assignment dropdowns).
+const WIDGET_LABELS = { photos: 'Photos', calendar: 'Calendar', whiteboard: 'Whiteboard (alpha)' };
+
 function showStatus(ok, message) {
   // While a <dialog> is open, its own .dialog-result container gets the
   // message instead of the page-level #result -- otherwise it lands