feat(dashboard): change UI font from the theme picker, independent of theme (#41145)

The dashboard font is now selectable from the UI, not just YAML. A new Font
section in the header theme picker overrides the UI font of whatever theme is
active; the choice is orthogonal to the theme and survives theme switches.
Each theme keeps its own font as the default — picking "Theme default" clears
the override.

- web/src/themes/fonts.ts: curated font catalog (system + Google Fonts across
  sans/serif/mono), each with a family stack and optional webfont URL. The
  catalog is the only injected-font surface — no free-text URL box, so the
  injected <link> origins stay fixed.
- web/src/themes/context.tsx: font-override state (localStorage + server),
  applied after theme typography so it wins; theme apply re-asserts it, and
  clearing re-runs theme apply to restore the theme's own font. Mono is left
  to the theme so code/terminal are untouched.
- web/src/components/ThemeSwitcher.tsx: Font section with grouped, self-
  previewing font rows and a "Theme default" clear option.
- hermes_cli/web_server.py: GET/PUT /api/dashboard/font persisting to
  config.yaml dashboard.font, with a server-side id allow-list (unknown ids
  coerce to the theme sentinel).
- i18n + types, api client methods, tests, and docs.

Validation: 6 new backend endpoint tests pass; tsc + vite build clean; live
browser test confirmed pick/persist/survive-theme-switch/clear all work.

web/src/i18n/en.ts

@@ -518,6 +518,12 @@ export const en: Translations = {
   theme: {
     title: "Theme",
     switchTheme: "Switch theme",
+    fontTitle: "Font",
+    fontDefault: "Theme default",
+    fontDefaultHint: "Use the active theme's font",
+    fontSans: "Sans",
+    fontSerif: "Serif",
+    fontMono: "Mono",
   },
 
   achievements: {

web/src/i18n/types.ts

@@ -539,6 +539,13 @@ export interface Translations {
   theme: {
     title: string;
     switchTheme: string;
+    /** Font-override section (optional — locales fall back to English). */
+    fontTitle?: string;
+    fontDefault?: string;
+    fontDefaultHint?: string;
+    fontSans?: string;
+    fontSerif?: string;
+    fontMono?: string;
   };
 
   // ── Achievements plugin (plugins/hermes-achievements) ──