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-core/src/prefs.rs

@@ -60,6 +60,100 @@ pub struct PlaybackPrefs {
     /// everything at 1.25, and a speed that reset every episode would be a
     /// setting nobody could use.
     pub speed: f64,
+    /// How plain subtitles look. Kept with the rest because it is the same
+    /// kind of choice: made once, for every file after.
+    pub subtitle_style: SubtitleStyle,
+}
+
+/// How plain-text subtitles are drawn.
+///
+/// Plain only, except for the size: an SRT track has no look of its own and
+/// gets mpv's, which is thin white text that a bright scene swallows. A styled
+/// (ASS) track was typeset by somebody, its signs placed and coloured by hand,
+/// and repainting it in one colour on one box undoes that work — so the
+/// backdrop and colour leave it alone, and only the scale reaches it.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Default, serde::Serialize, serde::Deserialize)]
+#[serde(default)]
+pub struct SubtitleStyle {
+    pub size: SubtitleSize,
+    pub backdrop: SubtitleBackdrop,
+    pub colour: SubtitleColour,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Default, serde::Serialize, serde::Deserialize)]
+#[serde(rename_all = "kebab-case")]
+pub enum SubtitleSize {
+    Small,
+    #[default]
+    Normal,
+    Large,
+    Larger,
+}
+
+impl SubtitleSize {
+    pub const ALL: [Self; 4] = [Self::Small, Self::Normal, Self::Large, Self::Larger];
+
+    pub fn label(self) -> &'static str {
+        match self {
+            Self::Small => "Small",
+            Self::Normal => "Normal",
+            Self::Large => "Large",
+            Self::Larger => "Larger",
+        }
+    }
+
+    /// Relative to mpv's own size, which is itself relative to the video's
+    /// height — so one setting reads the same on a phone and a television.
+    pub fn scale(self) -> f64 {
+        match self {
+            Self::Small => 0.8,
+            Self::Normal => 1.0,
+            Self::Large => 1.25,
+            Self::Larger => 1.5,
+        }
+    }
+}
+
+/// What keeps the text readable against the picture.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Default, serde::Serialize, serde::Deserialize)]
+#[serde(rename_all = "kebab-case")]
+pub enum SubtitleBackdrop {
+    /// A dark outline round each letter: mpv's own, and the default.
+    #[default]
+    Outline,
+    /// A dark, part-transparent box behind each line. Uglier, and the only
+    /// thing that holds up on snow, sky and white walls.
+    Box,
+}
+
+impl SubtitleBackdrop {
+    pub const ALL: [Self; 2] = [Self::Outline, Self::Box];
+
+    pub fn label(self) -> &'static str {
+        match self {
+            Self::Outline => "Outline",
+            Self::Box => "Dark box",
+        }
+    }
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Default, serde::Serialize, serde::Deserialize)]
+#[serde(rename_all = "kebab-case")]
+pub enum SubtitleColour {
+    #[default]
+    White,
+    Yellow,
+}
+
+impl SubtitleColour {
+    pub const ALL: [Self; 2] = [Self::White, Self::Yellow];
+
+    pub fn label(self) -> &'static str {
+        match self {
+            Self::White => "White",
+            Self::Yellow => "Yellow",
+        }
+    }
 }
 
 impl Default for PlaybackPrefs {
@@ -73,6 +167,7 @@ impl Default for PlaybackPrefs {
             autoplay_next: true,
             auto_skip: false,
             speed: 1.0,
+            subtitle_style: SubtitleStyle::default(),
         }
     }
 }
@@ -149,6 +244,11 @@ mod tests {
             autoplay_next: false,
             auto_skip: true,
             speed: 1.5,
+            subtitle_style: SubtitleStyle {
+                size: SubtitleSize::Larger,
+                backdrop: SubtitleBackdrop::Box,
+                colour: SubtitleColour::Yellow,
+            },
         };
         save(&dirs, &prefs).unwrap();
         assert_eq!(load(&dirs).unwrap(), prefs);
@@ -156,6 +256,14 @@ mod tests {
         std::fs::remove_dir_all(&temp).ok();
     }
 
+    #[test]
+    fn a_file_from_before_subtitle_styles_loads_with_the_default_style() {
+        let prefs: PlaybackPrefs =
+            serde_json::from_str(r#"{"volume": 60.0, "subtitles": true, "speed": 1.0}"#).unwrap();
+        assert_eq!(prefs.subtitle_style, SubtitleStyle::default());
+        assert_eq!(prefs.volume, 60.0);
+    }
+
     #[test]
     fn a_volume_no_player_could_honour_is_clamped_rather_than_passed_on() {
         let prefs = PlaybackPrefs {