feat(settings): add interface font selection

apps/desktop/src/lib/appFonts.ts

@@ -0,0 +1,6 @@
+export const APP_FONT_SANS_CSS_VAR = "--font-sans";
+
+export const DEFAULT_UI_FONT_FAMILY = `"Geist Variable", "PingFang SC", "Hiragino Sans GB", "Microsoft YaHei", "Segoe UI", system-ui, sans-serif`;
+
+// Native-feeling UI option without DBX's bundled/brand font at the front of the stack.
+export const SYSTEM_UI_FONT_FAMILY = `system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif`;