refactor(ui): Lucide migration unit 2 — Icons.* -> LucideIcons.* sweep (#60)

Mechanical sweep across 30 files: every Material Icons.* replaced with
the signed-off Lucide equivalent + a flutter_lucide import per file.
Zero Material Icons.* remain in lib/; no unused imports.

Judgment-call mappings: album->disc_3, library_music->library_big,
playlist_play->list_video, graphic_eq->audio_lines,
system_update->download, restore->archive_restore,
download_done->circle_check_big.

track_actions_sheet like menu row: collapsed `liked ? favorite :
favorite_border` to a single LucideIcons.heart (the row's Like/Unlike
text label conveys state). Icon-only LikeButton + the notification keep
the filled-vs-outline shape per the design decision.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

flutter_client/lib/admin/widgets/admin_user_row.dart

@@ -1,4 +1,5 @@
 import 'package:flutter/material.dart';
+import 'package:flutter_lucide/flutter_lucide.dart';
 
 import '../../models/admin_user.dart';
 import '../../theme/theme_extension.dart';
@@ -29,7 +30,7 @@ class AdminUserRow extends StatelessWidget {
             _Badge(label: 'auto-approve', color: fs.moss),
         ],
       ),
-      trailing: Icon(Icons.chevron_right, color: fs.ash),
+      trailing: Icon(LucideIcons.chevron_right, color: fs.ash),
       onTap: onTap,
     );
   }