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/templates/frame_config.html

@@ -53,6 +53,39 @@
           <button type="submit">Save</button>
         </form>
       </section>
+
+      <section class="card">
+        <h2 class="card-title">Button assignments</h2>
+        <p class="sub">What the frame's physical NEXT and BACK buttons do --
+          assign one or more widget actions to each, in the order they
+          should run. A button with several actions runs all of them, in
+          order, then the panel redraws once.</p>
+        <p class="sub" id="button-assign-empty-hint" style="display: none;">
+          Add a widget on the Layout tab first -- there's nothing to assign
+          a button to yet.</p>
+
+        <div id="button-assign-groups">
+          <div class="button-assign-group">
+            <h3 class="button-assign-label">NEXT button</h3>
+            <ul class="button-action-list" id="button-actions-next"></ul>
+            <div class="button-action-add">
+              <select id="button-add-widget-next"></select>
+              <select id="button-add-action-next"></select>
+              <button type="button" class="secondary btn-inline" id="button-add-next">Add</button>
+            </div>
+          </div>
+
+          <div class="button-assign-group" style="margin-top: 20px;">
+            <h3 class="button-assign-label">BACK button</h3>
+            <ul class="button-action-list" id="button-actions-back"></ul>
+            <div class="button-action-add">
+              <select id="button-add-widget-back"></select>
+              <select id="button-add-action-back"></select>
+              <button type="button" class="secondary btn-inline" id="button-add-back">Add</button>
+            </div>
+          </div>
+        </div>
+      </section>
     </div>
 
     <div class="side-col">