feat: add selectable interface and code fonts (10 for each)

Adds separate UI and code font choices in settings
Applies font changes immediately
Lazy-loads selected remote fonts

packages/ui/src/lib/fontLoader.ts

@@ -0,0 +1,59 @@
+import { CODE_FONT_OPTION_MAP, UI_FONT_OPTION_MAP, type FontFaceSource, type MonoFontOption, type UiFontOption } from '@/lib/fontOptions';
+
+const loadedFaces = new Set<string>();
+const pendingFaces = new Map<string, Promise<void>>();
+
+const buildFontUrl = (source: FontFaceSource, weight: number) => {
+  const packageName = encodeURIComponent(source.packageName).replace('%40', '@').replace('%2F', '/');
+  return `https://cdn.jsdelivr.net/npm/${packageName}/files/${source.filePrefix}-latin-${weight}-normal.woff2`;
+};
+
+const loadFace = (source: FontFaceSource, weight: number) => {
+  const key = `${source.family}:${weight}`;
+  if (loadedFaces.has(key)) {
+    return Promise.resolve();
+  }
+
+  const pending = pendingFaces.get(key);
+  if (pending) {
+    return pending;
+  }
+
+  if (typeof document === 'undefined' || typeof FontFace === 'undefined' || !document.fonts) {
+    return Promise.resolve();
+  }
+
+  const face = new FontFace(source.family, `url(${buildFontUrl(source, weight)}) format('woff2')`, {
+    style: 'normal',
+    weight: String(weight),
+    display: 'swap',
+  });
+
+  document.fonts.add(face);
+  const promise = face.load()
+    .then(() => {
+      loadedFaces.add(key);
+    })
+    .catch((error) => {
+      document.fonts.delete(face);
+      console.warn(`Failed to load font: ${source.family} ${weight}`, error);
+    })
+    .finally(() => {
+      pendingFaces.delete(key);
+    });
+
+  pendingFaces.set(key, promise);
+  return promise;
+};
+
+const loadSource = (source: FontFaceSource | undefined) => {
+  if (!source) {
+    return Promise.resolve();
+  }
+
+  return Promise.all(source.weights.map((weight) => loadFace(source, weight))).then(() => undefined);
+};
+
+export const loadUiFont = (font: UiFontOption) => loadSource(UI_FONT_OPTION_MAP[font]?.source);
+
+export const loadMonoFont = (font: MonoFontOption) => loadSource(CODE_FONT_OPTION_MAP[font]?.source);