feat(core,player,app): subtitle size, background and colour settings

Plain-text subtitles (SRT) were drawn in mpv's bare default, thin white
text that bright scenes swallow. Settings → Playback now picks a size,
an outline or a dark box, and white or yellow text. Styled (ASS) tracks
keep their own look and take only the size.

The box options are mpv 0.38's, so they are set best effort: an older
libmpv logs and plays on. The style syncs with the other playback
settings, and the Android bridge keeps the stored style instead of
resetting it on save.

crates/pstr-app/src/ui/settings.rs

@@ -7,7 +7,9 @@
 
 use pstr_core::appearance::{Accent, Appearance, Flavor, Tiles};
 use pstr_core::metadata::{MetadataConfig, ProviderId, TitleNames};
-use pstr_core::prefs::PlaybackPrefs;
+use pstr_core::prefs::{
+    PlaybackPrefs, SubtitleBackdrop, SubtitleColour, SubtitleSize, SubtitleStyle,
+};
 use pstr_player::LANGUAGES;
 
 use crate::app::Action;
@@ -315,6 +317,7 @@ fn playback_form(ui: &mut egui::Ui, prefs: Prefs<'_>, actions: &mut Vec<Action>)
         ),
         |ui| {
             languages(ui, prefs.playback, actions);
+            subtitle_style(ui, prefs.playback.subtitle_style, actions);
             let mut autoplay = prefs.autoplay;
             ui::widgets::settings_row(
                 ui,
@@ -428,6 +431,52 @@ fn languages(ui: &mut egui::Ui, playback: &PlaybackPrefs, actions: &mut Vec<Acti
     );
 }
 
+/// How plain subtitles are drawn. A styled track keeps its own look apart
+/// from the size; see [`pstr_core::prefs::SubtitleStyle`].
+fn subtitle_style(ui: &mut egui::Ui, style: SubtitleStyle, actions: &mut Vec<Action>) {
+    let mut chosen = style;
+    ui::widgets::settings_row(
+        ui,
+        "Subtitle size",
+        Some("Styled subtitles are scaled too. From the next file."),
+        |ui| {
+            egui::ComboBox::from_id_salt("subtitle-size")
+                .selected_text(chosen.size.label())
+                .show_ui(ui, |ui| {
+                    for size in SubtitleSize::ALL {
+                        ui.selectable_value(&mut chosen.size, size, size.label());
+                    }
+                });
+        },
+    );
+    ui::widgets::settings_row(
+        ui,
+        "Subtitle background",
+        Some("A box reads on bright scenes. Plain subtitles only; styled ones keep their own."),
+        |ui| {
+            egui::ComboBox::from_id_salt("subtitle-backdrop")
+                .selected_text(chosen.backdrop.label())
+                .show_ui(ui, |ui| {
+                    for backdrop in SubtitleBackdrop::ALL {
+                        ui.selectable_value(&mut chosen.backdrop, backdrop, backdrop.label());
+                    }
+                });
+        },
+    );
+    ui::widgets::settings_row(ui, "Subtitle colour", Some("Plain subtitles only."), |ui| {
+        egui::ComboBox::from_id_salt("subtitle-colour")
+            .selected_text(chosen.colour.label())
+            .show_ui(ui, |ui| {
+                for colour in SubtitleColour::ALL {
+                    ui.selectable_value(&mut chosen.colour, colour, colour.label());
+                }
+            });
+    });
+    if chosen != style {
+        actions.push(Action::SetSubtitleStyle(chosen));
+    }
+}
+
 /// The streaming cache: how big it may get, and a way to empty it.
 fn storage_form(ui: &mut egui::Ui, prefs: Prefs<'_>, actions: &mut Vec<Action>) {
     ui::widgets::settings_group(