Add Frame admin panel with full remote config

- New frame_config API endpoint (GET/POST) stores all frame display
  settings in frame-config.json on the BirdNET server
- Frame menu item in the admin drawer opens a full settings panel
  with sections: Display, Titles, Collage, Hardware, Source, Auth
- Changes auto-save via debounced POST; the frame Pi picks them up
  on its next 15-min timer cycle
- TODAY window button in the time picker with configurable start
  hour (localStorage bird:todayStart, adjustable in Settings)

Co-Authored-By: Claude Opus 4 <noreply@anthropic.com>

avian/frontend/styles.css

@@ -1622,3 +1622,15 @@
  * See avian/forwarding/. */
 body.av-local #dd-locked { display: none; }
 body.av-local #dd-items  { display: block; }
+
+  .frame-section-label {
+    font: 600 11px/1 var(--sans, ui-sans-serif, system-ui, sans-serif);
+    text-transform: uppercase;
+    letter-spacing: 0.05em;
+    color: var(--ink-soft, #8a8078);
+    padding: 18px 0 6px;
+    border-bottom: 1px solid rgba(26,22,18,0.08);
+    margin-bottom: 2px;
+  }
+  .frame-section-label:first-child { padding-top: 4px; }
+