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.

hermes_cli/web_server.py

@@ -9221,6 +9221,52 @@ async def set_dashboard_theme(body: ThemeSetBody):
     return {"ok": True, "theme": body.name}
 
 
+# Curated font-override ids. Kept in sync with FONT_CHOICES in
+# web/src/themes/fonts.ts — the frontend owns the stacks + webfont URLs;
+# the backend only needs the id allow-list so it can reject anything not
+# in the vetted catalog (the font's webfont URL is injected as a <link>,
+# so we never accept an arbitrary user-supplied id/URL here).
+_FONT_DEFAULT_ID = "theme"
+_FONT_CHOICES = frozenset({
+    "system-sans", "system-serif", "system-mono",
+    "inter", "ibm-plex-sans", "work-sans", "atkinson-hyperlegible", "dm-sans",
+    "spectral", "fraunces", "source-serif",
+    "jetbrains-mono", "ibm-plex-mono", "space-mono",
+})
+
+
+@app.get("/api/dashboard/font")
+async def get_dashboard_font():
+    """Return the active font override (``"theme"`` = use the theme's font)."""
+    config = load_config()
+    font = cfg_get(config, "dashboard", "font", default=_FONT_DEFAULT_ID)
+    if font not in _FONT_CHOICES:
+        font = _FONT_DEFAULT_ID
+    return {"font": font}
+
+
+class FontSetBody(BaseModel):
+    font: str
+
+
+@app.put("/api/dashboard/font")
+async def set_dashboard_font(body: FontSetBody):
+    """Set the dashboard font override (persists to config.yaml).
+
+    Accepts any id in the curated catalog, or ``"theme"`` to clear the
+    override and fall back to the active theme's own font. Unknown ids are
+    coerced to ``"theme"`` rather than 400'd so a stale client can't wedge
+    the picker.
+    """
+    font = body.font if body.font in _FONT_CHOICES else _FONT_DEFAULT_ID
+    config = load_config()
+    if "dashboard" not in config:
+        config["dashboard"] = {}
+    config["dashboard"]["font"] = font
+    save_config(config)
+    return {"ok": True, "font": font}
+
+
 # ---------------------------------------------------------------------------
 # Dashboard plugin system
 # ---------------------------------------------------------------------------