feat: add home feed provider setting, fix Qobuz cover URL propagation

- Add homeFeedProvider field to AppSettings with picker UI in extensions page
- Update explore_provider to respect user's home feed provider preference
- Add normalizeCoverReference() and normalizeRemoteHttpUrl() to filter
  invalid cover URLs (no scheme, no host, protocol-relative)
- Apply cover URL normalization across all screens and providers to
  prevent 'no host specified in URI' errors from Qobuz
- Propagate CoverURL from QobuzDownloadResult through Go backend so
  cover art is available even when request metadata is incomplete

lib/utils/string_utils.dart

@@ -6,6 +6,41 @@ String? normalizeOptionalString(String? value) {
   return trimmed;
 }
 
+final RegExp _windowsAbsolutePathPattern = RegExp(r'^[A-Za-z]:[\\/]');
+
+bool _looksLikeLocalReference(String value) {
+  return value.startsWith('/') ||
+      value.startsWith('content://') ||
+      value.startsWith('file://') ||
+      _windowsAbsolutePathPattern.hasMatch(value);
+}
+
+String? normalizeCoverReference(String? value) {
+  final normalized = normalizeOptionalString(value);
+  if (normalized == null) return null;
+
+  if (normalized.startsWith('//')) {
+    return 'https:$normalized';
+  }
+
+  if (normalized.startsWith('http://') ||
+      normalized.startsWith('https://') ||
+      _looksLikeLocalReference(normalized)) {
+    return normalized;
+  }
+
+  return null;
+}
+
+String? normalizeRemoteHttpUrl(String? value) {
+  final normalized = normalizeCoverReference(value);
+  if (normalized == null) return null;
+  if (normalized.startsWith('http://') || normalized.startsWith('https://')) {
+    return normalized;
+  }
+  return null;
+}
+
 String formatSampleRateKHz(int sampleRate) {
   final khz = sampleRate / 1000;
   final precision = sampleRate % 1000 == 0 ? 0 : 1;