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/lib/components/StatusBar.svelte

@@ -113,6 +113,8 @@
     background_image_opacity: 0.3,
     custom_font_path: null,
     custom_font_family: null,
+    custom_ui_font_path: null,
+    custom_ui_font_family: null,
   });
 
   let streamerModeActive = $state(false);