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/admin_landing_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';
 
@@ -34,7 +35,7 @@ class AdminLandingScreen extends ConsumerWidget {
               children: [
                 AdminSectionCard(
                   key: const Key('admin_card_requests'),
-                  icon: Icons.inbox,
+                  icon: LucideIcons.inbox,
                   title: 'Requests',
                   subtitle: 'Approve or reject Lidarr requests',
                   count: c.requests,
@@ -42,7 +43,7 @@ class AdminLandingScreen extends ConsumerWidget {
                 ),
                 AdminSectionCard(
                   key: const Key('admin_card_quarantine'),
-                  icon: Icons.warning_amber,
+                  icon: LucideIcons.triangle_alert,
                   title: 'Quarantine',
                   subtitle: 'Resolve scan failures',
                   count: c.quarantine,
@@ -50,7 +51,7 @@ class AdminLandingScreen extends ConsumerWidget {
                 ),
                 AdminSectionCard(
                   key: const Key('admin_card_users'),
-                  icon: Icons.people,
+                  icon: LucideIcons.users,
                   title: 'Users',
                   subtitle: 'Manage accounts and invites',
                   count: c.users,