Preferences: choose the interface font (#1408)

Adds an Interface font setting that leads the proportional family, with the default face and CJK/Hangul fallbacks kept behind it (#1290).

Co-authored-by: DuBearte <DuBearte@users.noreply.github.com>
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

crates/ui-egui/src/lib.rs

@@ -374,6 +374,8 @@ pub struct SolveApp {
     /// The language the installed font fallback chain is ordered for (`fonts::fallback_order`);
     /// changing `preferences.language` re-installs the fonts on the next frame.
     applied_fonts_language: String,
+    /// The interface font the fonts were last set up with (`Prefs::ui_font`).
+    applied_ui_font: String,
 }
 
 impl SolveApp {
@@ -456,6 +458,7 @@ impl SolveApp {
             file_load: file_load::FileLoad::default(),
             docs: documents::Documents::default(),
             applied_fonts_language: "en".into(),
+            applied_ui_font: String::new(),
         }
     }
 
@@ -1273,9 +1276,10 @@ impl SolveApp {
     pub fn ui(&mut self, ui: &mut egui::Ui) {
         expression_complete::begin_frame(ui.ctx());
         let t0 = now_ms();
-        if self.preferences.language != self.applied_fonts_language {
+        if self.preferences.language != self.applied_fonts_language || self.preferences.ui_font != self.applied_ui_font {
             self.applied_fonts_language.clone_from(&self.preferences.language);
-            crate::fonts::setup_fonts_for(ui.ctx(), &self.preferences.language);
+            self.applied_ui_font.clone_from(&self.preferences.ui_font);
+            crate::fonts::setup_fonts_for(ui.ctx(), &self.preferences.language, &self.preferences.ui_font);
         }
         let _language = crate::i18n::scope(&self.preferences.language);
         let mut report_input = None;