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.

website/docs/user-guide/features/web-dashboard.md

@@ -1023,6 +1023,8 @@ The dashboard ships with six built-in themes and can be extended with user-defin
 
 **Switch themes live** from the header bar — click the palette icon next to the language switcher. Selection persists to `config.yaml` under `dashboard.theme` and is restored on page load.
 
+**Change the font independently** from the same picker — the **Font** section below the theme list overrides the UI font of whatever theme is active. The choice persists across theme switches (`config.yaml` → `dashboard.font`); pick **Theme default** to clear it and return to the active theme's own font.
+
 Built-in themes:
 
 | Theme | Character |