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/api/menu.php

@@ -31,6 +31,7 @@ if (getenv('AV_REQUIRE_AUTH') === '1' && empty($_SERVER['HTTP_AUTHORIZATION']))
 echo json_encode([
     'items' => [
         ['label' => 'settings', 'href' => '/#admin=settings', 'native' => true],
+        ['label' => 'frame',    'href' => '/#admin=frame',    'native' => true],
         ['label' => 'system',   'href' => '/#admin=system',   'native' => true],
         ['label' => 'logs',     'href' => '/#admin=logs',     'native' => true],
         ['label' => 'tools',    'href' => '/#admin=tools',    'native' => true],