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/playlists/playlist_detail_screen.dart

@@ -1,4 +1,5 @@
 import 'package:flutter/material.dart';
+import 'package:flutter_lucide/flutter_lucide.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:go_router/go_router.dart';
 
@@ -54,7 +55,7 @@ class PlaylistDetailScreen extends ConsumerWidget {
         backgroundColor: fs.obsidian,
         elevation: 0,
         leading: IconButton(
-          icon: Icon(Icons.arrow_back, color: fs.parchment),
+          icon: Icon(LucideIcons.arrow_left, color: fs.parchment),
           onPressed: () => context.pop(),
         ),
         title: headerName.isEmpty
@@ -268,7 +269,7 @@ class _Header extends ConsumerWidget {
               OutlinedButton.icon(
                 key: const Key('regenerate_playlist_button'),
                 onPressed: () => _regenerate(context, ref),
-                icon: const Icon(Icons.refresh, size: 16),
+                icon: const Icon(LucideIcons.refresh_cw, size: 16),
                 label: const Text('Regenerate'),
               ),
               const SizedBox(width: 8),
@@ -281,7 +282,7 @@ class _Header extends ConsumerWidget {
                       source: p.refreshable ? p.systemVariant : null,
                     );
               },
-              icon: const Icon(Icons.play_arrow),
+              icon: const Icon(LucideIcons.play),
               label: const Text('Play'),
               style: FilledButton.styleFrom(
                 backgroundColor: fs.accent,