feat(ui): add shared widgets, utils, and models for deduplicated screens

Single homes for logic that was copy-pasted across screens: the settings
collapsing header, album detail header (online screen's design as the
reference), selection pill button + bottom-bar chrome, disc separator
chip, error card, cover URL upgrade, byte/clock formatting, duration
extraction, UnifiedLibraryItem (moved out of the queue_tab library so
other screens can import it), and the batch convert/ReplayGain engine
keyed on it.

lib/utils/cover_art_utils.dart

@@ -0,0 +1,22 @@
+/// Deezer CDN cover size pattern: /WxH-q-p-o-n.jpg
+final RegExp _deezerCoverSizeRegex = RegExp(
+  r'/(\d+)x(\d+)-(\d+)-(\d+)-(\d+)-(\d+)\.jpg$',
+);
+
+/// Upgrades a Spotify/Deezer cover URL to a display-quality resolution
+/// (Spotify 300px → 640px, Deezer → 1000x1000 preserving quality params).
+/// Non-matching URLs pass through unchanged.
+String? highResCoverUrl(String? url) {
+  if (url == null) return null;
+  if (url.contains('ab67616d00001e02')) {
+    return url.replaceAll('ab67616d00001e02', 'ab67616d0000b273');
+  }
+  if (url.contains('cdn-images.dzcdn.net') &&
+      _deezerCoverSizeRegex.hasMatch(url)) {
+    return url.replaceAllMapped(
+      _deezerCoverSizeRegex,
+      (m) => '/1000x1000-${m[3]}-${m[4]}-${m[5]}-${m[6]}.jpg',
+    );
+  }
+  return url;
+}

lib/utils/int_utils.dart

@@ -11,3 +11,29 @@ int? readPositiveInt(dynamic value) {
   if (parsed == null || parsed <= 0) return null;
   return parsed;
 }
+
+int extractDurationMs(Map<String, dynamic> data) {
+  final durationMsRaw = data['duration_ms'];
+  if (durationMsRaw is num && durationMsRaw > 0) {
+    return durationMsRaw.toInt();
+  }
+  if (durationMsRaw is String) {
+    final parsed = num.tryParse(durationMsRaw.trim());
+    if (parsed != null && parsed > 0) {
+      return parsed.toInt();
+    }
+  }
+
+  final durationSecRaw = data['duration'];
+  if (durationSecRaw is num && durationSecRaw > 0) {
+    return (durationSecRaw * 1000).toInt();
+  }
+  if (durationSecRaw is String) {
+    final parsed = num.tryParse(durationSecRaw.trim());
+    if (parsed != null && parsed > 0) {
+      return (parsed * 1000).toInt();
+    }
+  }
+
+  return 0;
+}

lib/utils/string_utils.dart

@@ -55,6 +55,24 @@ String? normalizeRemoteHttpUrl(String? value) {
   return null;
 }
 
+/// Human-readable byte size: "512 B", "3.4 KB", "12.0 MB", "1.25 GB".
+String formatBytes(int bytes) {
+  if (bytes < 1024) return '$bytes B';
+  if (bytes < 1024 * 1024) return '${(bytes / 1024).toStringAsFixed(1)} KB';
+  if (bytes < 1024 * 1024 * 1024) {
+    return '${(bytes / (1024 * 1024)).toStringAsFixed(1)} MB';
+  }
+  return '${(bytes / (1024 * 1024 * 1024)).toStringAsFixed(2)} GB';
+}
+
+/// "m:ss" clock formatting for track durations.
+String formatClock(num seconds) {
+  final total = seconds.round();
+  final minutes = total ~/ 60;
+  final secs = total % 60;
+  return '$minutes:${secs.toString().padLeft(2, '0')}';
+}
+
 String formatSampleRateKHz(int sampleRate) {
   final khz = sampleRate / 1000;
   final precision = sampleRate % 1000 == 0 ? 0 : 1;