feat: add per-project font override settings

Add font selectors that allow per-project font customization for both
sans and mono fonts, independent of theme selection. Uses system fonts.

- Add fontFamilySans and fontFamilyMono to ProjectSettings and Project types
- Create ui-font-options.ts config with system font options
- Add store actions: setProjectFontSans, setProjectFontMono, getEffectiveFontSans, getEffectiveFontMono
- Apply font CSS variables in root component
- Add font selector UI in project-theme-section (Project Settings → Theme)

apps/ui/src/config/ui-font-options.ts

@@ -0,0 +1,54 @@
+/**
+ * Font options for per-project font customization
+ *
+ * These are system fonts (no bundled @fontsource packages required).
+ * Users must have the fonts installed on their system for them to work.
+ */
+
+export interface UIFontOption {
+  value: string; // CSS font-family value (empty string means "use default")
+  label: string; // Display label for the dropdown
+}
+
+/**
+ * Sans/UI fonts for headings, labels, and general text
+ *
+ * Empty value means "use the theme default" (Geist Sans for all themes)
+ */
+export const UI_SANS_FONT_OPTIONS: readonly UIFontOption[] = [
+  { value: '', label: 'Default (Geist Sans)' },
+  { value: "'Inter', system-ui, sans-serif", label: 'Inter' },
+  { value: "'SF Pro', system-ui, sans-serif", label: 'SF Pro' },
+  { value: "'Source Sans 3', system-ui, sans-serif", label: 'Source Sans' },
+  { value: "'IBM Plex Sans', system-ui, sans-serif", label: 'IBM Plex Sans' },
+  { value: "'Roboto', system-ui, sans-serif", label: 'Roboto' },
+  { value: 'system-ui, sans-serif', label: 'System Default' },
+] as const;
+
+/**
+ * Mono/code fonts for code blocks, terminals, and monospaced text
+ *
+ * Empty value means "use the theme default" (Geist Mono for all themes)
+ */
+export const UI_MONO_FONT_OPTIONS: readonly UIFontOption[] = [
+  { value: '', label: 'Default (Geist Mono)' },
+  { value: "'JetBrains Mono', monospace", label: 'JetBrains Mono' },
+  { value: "'Fira Code', monospace", label: 'Fira Code' },
+  { value: "'SF Mono', Menlo, Monaco, monospace", label: 'SF Mono' },
+  { value: "'Source Code Pro', monospace", label: 'Source Code Pro' },
+  { value: "'IBM Plex Mono', monospace", label: 'IBM Plex Mono' },
+  { value: "Menlo, Monaco, 'Courier New', monospace", label: 'Menlo / Monaco' },
+  { value: "'Cascadia Code', monospace", label: 'Cascadia Code' },
+] as const;
+
+/**
+ * Get the display label for a font value
+ */
+export function getFontLabel(
+  fontValue: string | undefined,
+  options: readonly UIFontOption[]
+): string {
+  if (!fontValue) return options[0].label;
+  const option = options.find((o) => o.value === fontValue);
+  return option?.label ?? fontValue;
+}