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/discover/discover_screen.dart

@@ -1,6 +1,7 @@
 import 'package:cached_network_image/cached_network_image.dart';
 import 'package:dio/dio.dart';
 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';
 
@@ -167,7 +168,7 @@ class _DiscoverScreenState extends ConsumerState<DiscoverScreen> {
         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: Text('Discover', style: TextStyle(color: fs.parchment)),
@@ -193,7 +194,7 @@ class _DiscoverScreenState extends ConsumerState<DiscoverScreen> {
                     borderSide: BorderSide(color: fs.accent),
                   ),
                   suffixIcon: IconButton(
-                    icon: Icon(Icons.search, color: fs.ash),
+                    icon: Icon(LucideIcons.search, color: fs.ash),
                     onPressed: _runSearch,
                   ),
                 ),
@@ -336,14 +337,14 @@ class _ResultTile extends StatelessWidget {
             height: 56,
             color: fs.slate,
             child: row.imageUrl.isEmpty
-                ? Icon(Icons.album, color: fs.ash)
+                ? Icon(LucideIcons.disc_3, color: fs.ash)
                 : CachedNetworkImage(
                     imageUrl: row.imageUrl,
                     fit: BoxFit.cover,
                     fadeInDuration: const Duration(milliseconds: 120),
                     fadeOutDuration: Duration.zero,
                     errorWidget: (_, __, ___) =>
-                        Icon(Icons.album, color: fs.ash),
+                        Icon(LucideIcons.disc_3, color: fs.ash),
                   ),
           ),
         ),
@@ -419,14 +420,14 @@ class _SuggestionTile extends StatelessWidget {
             height: 56,
             color: fs.slate,
             child: s.imageUrl.isEmpty
-                ? Icon(Icons.person, color: fs.ash)
+                ? Icon(LucideIcons.user, color: fs.ash)
                 : CachedNetworkImage(
                     imageUrl: s.imageUrl,
                     fit: BoxFit.cover,
                     fadeInDuration: const Duration(milliseconds: 120),
                     fadeOutDuration: Duration.zero,
                     errorWidget: (_, __, ___) =>
-                        Icon(Icons.person, color: fs.ash),
+                        Icon(LucideIcons.user, color: fs.ash),
                   ),
           ),
         ),