feat: add custom UI font support

Allow users to specify a custom font for the entire app interface
(menus, labels, buttons) separately from the terminal font. Supports
Google Fonts URLs, direct font file URLs, and local file paths.

- Add custom_ui_font_path and custom_ui_font_family to Rust config
- Refactor applyCustomFont into shared applyFontFromSource helper
- Add applyCustomUiFont function using --ui-font-family CSS variable
- Update app.css to use --ui-font-family with fallback
- Apply custom UI font on startup in +page.svelte
- Add Custom UI Font section to ConfigSidebar settings panel
- Add tests for applyCustomUiFont and setCustomUiFont

src-tauri/src/config.rs

@@ -145,12 +145,19 @@ pub struct HikariConfig {
     #[serde(default)]
     pub show_thinking_blocks: bool,
 
-    // Custom font settings
+    // Custom terminal font settings
     #[serde(default)]
     pub custom_font_path: Option<String>,
 
     #[serde(default)]
     pub custom_font_family: Option<String>,
+
+    // Custom UI font settings
+    #[serde(default)]
+    pub custom_ui_font_path: Option<String>,
+
+    #[serde(default)]
+    pub custom_ui_font_family: Option<String>,
 }
 
 impl Default for HikariConfig {
@@ -192,6 +199,8 @@ impl Default for HikariConfig {
             show_thinking_blocks: false,
             custom_font_path: None,
             custom_font_family: None,
+            custom_ui_font_path: None,
+            custom_ui_font_family: None,
         }
     }
 }
@@ -309,6 +318,8 @@ mod tests {
         assert!(!config.show_thinking_blocks);
         assert!(config.custom_font_path.is_none());
         assert!(config.custom_font_family.is_none());
+        assert!(config.custom_ui_font_path.is_none());
+        assert!(config.custom_ui_font_family.is_none());
     }
 
     #[test]
@@ -350,6 +361,8 @@ mod tests {
             show_thinking_blocks: true,
             custom_font_path: Some("/home/naomi/.fonts/MyFont.ttf".to_string()),
             custom_font_family: Some("MyFont".to_string()),
+            custom_ui_font_path: None,
+            custom_ui_font_family: None,
         };
 
         let json = serde_json::to_string(&config).unwrap();