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/lib/api.ts

@@ -741,6 +741,14 @@ export const api = {
       headers: { "Content-Type": "application/json" },
       body: JSON.stringify({ name }),
     }),
+  getFontPref: () =>
+    fetchJSON<DashboardFontResponse>("/api/dashboard/font"),
+  setFontPref: (font: string) =>
+    fetchJSON<{ ok: boolean; font: string }>("/api/dashboard/font", {
+      method: "PUT",
+      headers: { "Content-Type": "application/json" },
+      body: JSON.stringify({ font }),
+    }),
 
   // ── Admin: MCP servers ──────────────────────────────────────────────
   getMcpServers: () => fetchJSON<{ servers: McpServer[] }>("/api/mcp/servers"),
@@ -1857,6 +1865,11 @@ export interface DashboardThemesResponse {
   themes: DashboardThemeSummary[];
 }
 
+export interface DashboardFontResponse {
+  /** Active font-override id, or "theme" when no override is set. */
+  font: string;
+}
+
 // ── Dashboard plugin types ─────────────────────────────────────────────
 
 export interface PluginManifestResponse {