fix(app): the subtitle preview is drawn in the chosen font

The preview drew every font in the app's own Inter, so picking one changed
nothing on screen. On Linux it now asks fontconfig for the face libass would
use — the family with the weight and slant wanted, or fontconfig's fallback
when it is not installed — loads it on a thread of its own and draws the
sample in it. A bold or italic the face lacks is made up, as libass does.
Windows and macOS have no fc-match and keep the app's font, as the hint there
still says.

The CJK fallback and the subtitle face are now both kept, so installing one
no longer drops the other.

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

@@ -463,7 +463,11 @@ fn subtitle_form(ui: &mut egui::Ui, style: &SubtitleStyle, actions: &mut Vec<Act
             ui::widgets::settings_row(
                 ui,
                 "Font",
-                Some("Any font installed here. The sample above is drawn in the app's own font."),
+                Some(if cfg!(target_os = "linux") {
+                    "Any font installed here."
+                } else {
+                    "Any font installed here. The sample above is drawn in the app's own font."
+                }),
                 |ui| {
                     ui::widgets::toggle(ui, &mut chosen.italic, "Italic");
                     ui::widgets::toggle(ui, &mut chosen.bold, "Bold");
@@ -662,9 +666,11 @@ fn font_picker(ui: &mut egui::Ui, font: &mut Option<String>) {
 /// A sample line, half on a bright sky and half on night, drawn in the
 /// chosen style — so whether it reads on both is visible before a film is.
 ///
-/// An approximation: the app's own font rather than the chosen one, egui's
-/// slant for italic, and no blur. Proportions follow mpv's, where sizes are
-/// in pixels of a 38-pixel font.
+/// An approximation. The font is the face fontconfig picks for the chosen one,
+/// as libass's is, but only on Linux; elsewhere, and while it loads, it is the
+/// app's own. A bold or italic the face lacks is made up, as libass does, and
+/// there is no blur. Proportions follow mpv's, where sizes are in pixels of a
+/// 38-pixel font.
 fn subtitle_preview(ui: &mut egui::Ui, style: &SubtitleStyle) {
     const HEIGHT: f32 = 132.0;
     /// mpv's `sub-font-size`, the unit every other size is relative to.
@@ -694,17 +700,27 @@ fn subtitle_preview(ui: &mut egui::Ui, style: &SubtitleStyle) {
 
     let font_px = 22.0 * style.scale as f32;
     let unit = font_px / MPV_FONT;
-    let family = if style.bold {
-        theme::semibold()
-    } else {
-        egui::FontFamily::Proportional
+    let face = theme::subtitle_face(
+        ui.ctx(),
+        style.font.as_deref().unwrap_or(SubtitleStyle::DEFAULT_FONT),
+        style.bold,
+        style.italic,
+    );
+    let (family, made_up_bold, made_up_italic) = match face {
+        Some(face) => (
+            theme::subtitle_family(),
+            style.bold && !face.bold,
+            style.italic && !face.italic,
+        ),
+        None if style.bold => (theme::semibold(), false, style.italic),
+        None => (egui::FontFamily::Proportional, false, style.italic),
     };
     let job = egui::text::LayoutJob::single_section(
         "Can you read this on both sides?".to_owned(),
         egui::TextFormat {
             font_id: egui::FontId::new(font_px, family),
             color: egui::Color32::PLACEHOLDER,
-            italics: style.italic,
+            italics: made_up_italic,
             ..Default::default()
         },
     );
@@ -763,6 +779,15 @@ fn subtitle_preview(ui: &mut egui::Ui, style: &SubtitleStyle) {
             }
         }
     }
+    if made_up_bold {
+        // libass thickens a face with no bold by a stroke of its own colour.
+        let thicken = (font_px / 40.0).max(0.5);
+        painter.galley_with_override_text_color(
+            at + egui::vec2(thicken, 0.0),
+            galley.clone(),
+            colour(style.colour),
+        );
+    }
     painter.galley_with_override_text_color(at, galley, colour(style.colour));
 }