feat: configurable intro/credits marker regex patterns

Add settings to customize regex patterns for matching intro and credits
markers in chapter titles, with the current hardcoded values as defaults.
Also reclassifies real markers with non-standard type strings against the
configured patterns.

lib/screens/settings/settings_screen.dart

@@ -98,6 +98,8 @@ class _SettingsScreenState extends State<SettingsScreen> with FocusableTab {
   static const _kAutoSkipIntro = 'auto_skip_intro';
   static const _kAutoSkipCredits = 'auto_skip_credits';
   static const _kAutoSkipDelay = 'auto_skip_delay';
+  static const _kIntroPattern = 'intro_pattern';
+  static const _kCreditsPattern = 'credits_pattern';
   static const _kDownloadLocation = 'download_location';
   static const _kDownloadOnWifiOnly = 'download_on_wifi_only';
   static const _kVideoPlayerControls = 'video_player_controls';
@@ -125,6 +127,8 @@ class _SettingsScreenState extends State<SettingsScreen> with FocusableTab {
   bool _autoSkipIntro = false;
   bool _autoSkipCredits = false;
   int _autoSkipDelay = 5;
+  String _introPattern = settings.SettingsService.defaultIntroPattern;
+  String _creditsPattern = settings.SettingsService.defaultCreditsPattern;
   bool _downloadOnWifiOnly = false;
   bool _videoPlayerNavigationEnabled = false;
   int _maxVolume = 100;
@@ -202,6 +206,8 @@ class _SettingsScreenState extends State<SettingsScreen> with FocusableTab {
       _autoSkipIntro = _settingsService.getAutoSkipIntro();
       _autoSkipCredits = _settingsService.getAutoSkipCredits();
       _autoSkipDelay = _settingsService.getAutoSkipDelay();
+      _introPattern = _settingsService.getIntroPattern();
+      _creditsPattern = _settingsService.getCreditsPattern();
       _downloadOnWifiOnly = _settingsService.getDownloadOnWifiOnly();
       _videoPlayerNavigationEnabled = _settingsService.getVideoPlayerNavigationEnabled();
       _maxVolume = _settingsService.getMaxVolume();
@@ -874,6 +880,38 @@ class _SettingsScreenState extends State<SettingsScreen> with FocusableTab {
             trailing: const AppIcon(Symbols.chevron_right_rounded, fill: 1),
             onTap: () => _showAutoSkipDelayDialog(),
           ),
+          ListTile(
+            focusNode: _focusTracker.get(_kIntroPattern),
+            leading: const AppIcon(Symbols.match_case_rounded, fill: 1),
+            title: Text(t.settings.introPattern),
+            subtitle: Text(t.settings.introPatternDescription),
+            trailing: const AppIcon(Symbols.chevron_right_rounded, fill: 1),
+            onTap: () => _showTextInputDialog(
+              title: t.settings.introPattern,
+              currentValue: _introPattern,
+              defaultValue: settings.SettingsService.defaultIntroPattern,
+              onSave: (value) async {
+                setState(() => _introPattern = value);
+                await _settingsService.setIntroPattern(value);
+              },
+            ),
+          ),
+          ListTile(
+            focusNode: _focusTracker.get(_kCreditsPattern),
+            leading: const AppIcon(Symbols.match_case_rounded, fill: 1),
+            title: Text(t.settings.creditsPattern),
+            subtitle: Text(t.settings.creditsPatternDescription),
+            trailing: const AppIcon(Symbols.chevron_right_rounded, fill: 1),
+            onTap: () => _showTextInputDialog(
+              title: t.settings.creditsPattern,
+              currentValue: _creditsPattern,
+              defaultValue: settings.SettingsService.defaultCreditsPattern,
+              onSave: (value) async {
+                setState(() => _creditsPattern = value);
+                await _settingsService.setCreditsPattern(value);
+              },
+            ),
+          ),
         ],
       ),
     );
@@ -1602,6 +1640,69 @@ class _SettingsScreenState extends State<SettingsScreen> with FocusableTab {
     );
   }
 
+  void _showTextInputDialog({
+    required String title,
+    required String currentValue,
+    required String defaultValue,
+    required Future<void> Function(String value) onSave,
+  }) {
+    final controller = TextEditingController(text: currentValue);
+    String? errorText;
+    final saveFocusNode = FocusNode();
+
+    showDialog(
+      context: context,
+      builder: (BuildContext dialogContext) {
+        return StatefulBuilder(
+          builder: (context, setDialogState) {
+            return AlertDialog(
+              title: Text(title),
+              content: TextField(
+                controller: controller,
+                decoration: InputDecoration(
+                  labelText: 'Regex',
+                  errorText: errorText,
+                ),
+                autofocus: true,
+                textInputAction: TextInputAction.done,
+                onEditingComplete: () => saveFocusNode.requestFocus(),
+                onChanged: (value) {
+                  setDialogState(() {
+                    try {
+                      RegExp(value, caseSensitive: false);
+                      errorText = null;
+                    } catch (_) {
+                      errorText = t.settings.invalidRegex;
+                    }
+                  });
+                },
+              ),
+              actions: [
+                TextButton(
+                  onPressed: () {
+                    controller.text = defaultValue;
+                    setDialogState(() => errorText = null);
+                  },
+                  child: Text(t.settings.resetToDefault),
+                ),
+                TextButton(onPressed: () => Navigator.pop(dialogContext), child: Text(t.common.cancel)),
+                TextButton(
+                  focusNode: saveFocusNode,
+                  onPressed: () async {
+                    if (errorText != null) return;
+                    await onSave(controller.text);
+                    if (dialogContext.mounted) Navigator.pop(dialogContext);
+                  },
+                  child: Text(t.common.save),
+                ),
+              ],
+            );
+          },
+        );
+      },
+    ).then((_) => saveFocusNode.dispose());
+  }
+
   void _showMaxVolumeDialog() {
     _showNumericInputDialog(
       title: t.settings.maxVolume,