feat(subtitles): optionally anchor text subtitles to the screen bottom

Adds an "Anchor to Screen" toggle under Subtitle Styling (Android +
ExoPlayer only, default off). When enabled, the text SubtitleView is
sized to the full container instead of the letterboxed video rect, so
SRT/VTT/mov_text cues render in the black bars below widescreen video
and font size and the position setting become relative to the physical
screen height. Bitmap (PGS/VOB) and ASS/libass rendering are unchanged;
mpv already places plaintext subtitles in the margins by default.

close #1730

lib/services/settings_service.dart

@@ -377,6 +377,12 @@ class SettingsService extends BaseSharedPreferencesService {
   );
   static const subtitleBold = BoolPref('subtitle_bold');
   static const subtitleItalic = BoolPref('subtitle_italic');
+
+  /// Render text subtitles (SRT/VTT/mov_text) anchored to the physical screen
+  /// instead of the video rect, so they land in the letterbox bars of
+  /// widescreen video (#1730). ExoPlayer backend only; mpv already places
+  /// plaintext subtitles in the margins by default (sub-use-margins=yes).
+  static const subtitleAnchorToScreen = BoolPref('subtitle_anchor_to_screen');
   static const cleanedOldImageCache = BoolPref('cleaned_old_image_cache');
   static const rememberTrackSelections = BoolPref('remember_track_selections', defaultValue: true);
 
@@ -935,6 +941,7 @@ class SettingsService extends BaseSharedPreferencesService {
     maxVolume,
     downmixCenterBoost,
     subtitlePosition,
+    subtitleAnchorToScreen,
     defaultPlaybackSpeed,
     defaultBoxFitMode,
     themeMode,