feat(app): design tokens, settings page, backdrop layering

Type/shape/motion tokens live in theme.rs and every ui/ module reads them
instead of scattering literal sizes and radii. Appearance and playback
preferences move out of the shares page into their own Settings page and
nav tab.

The title backdrop's veil and fades were painted after the header was laid
out, so they covered the name and overview as well as the still — the text
was being dimmed by the thing meant to make it readable. All band layers
now go into the shape slot reserved before the header, behind the text.

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

@@ -49,8 +49,8 @@ pub fn show(
             ui.set_width(620.0);
 
             ui.label(
-                egui::RichText::new(format!("Match “{}”", matcher.title_name))
-                    .size(18.0)
+                theme::Role::Heading
+                    .rich(format!("Match “{}”", matcher.title_name))
                     .strong()
                     .color(theme::text()),
             );
@@ -66,7 +66,7 @@ pub fn show(
             ui.add_space(10.0);
 
             if let Some(error) = &matcher.error {
-                ui.label(egui::RichText::new(error).size(12.0).color(theme::danger()));
+                ui.label(theme::Role::Caption.rich(error).color(theme::danger()));
                 ui.add_space(6.0);
             }
 
@@ -212,8 +212,8 @@ fn row(ui: &mut egui::Ui, art: &mut Art<'_>, found: &TitleMetadata) -> egui::Res
                 ui.add_space(10.0);
                 ui.vertical(|ui| {
                     ui.label(
-                        egui::RichText::new(&found.name)
-                            .size(14.0)
+                        theme::Role::Body
+                            .rich(&found.name)
                             .strong()
                             .color(theme::text()),
                     );
@@ -224,7 +224,7 @@ fn row(ui: &mut egui::Ui, art: &mut Art<'_>, found: &TitleMetadata) -> egui::Res
                     if let Some(overview) = &found.overview {
                         let mut job = egui::text::LayoutJob::simple(
                             overview.replace('\n', " "),
-                            egui::FontId::proportional(11.5),
+                            theme::Role::Caption.font(),
                             theme::muted(),
                             ui.available_width(),
                         );