feat: dashboard theme system with live switching

Add a theme engine for the web dashboard that mirrors the CLI skin
engine philosophy — pure data, no code changes needed for new themes.

Frontend:
- ThemeProvider context that loads active theme from backend on mount
  and applies CSS variable overrides to document.documentElement
- ThemeSwitcher dropdown component in the header (next to language
  switcher) with instant preview on click
- 6 built-in themes: Hermes Teal (default), Midnight, Ember, Mono,
  Cyberpunk, Rosé — each defines all 21 color tokens + overlay settings
- Theme types, presets, and context in web/src/themes/

Backend:
- GET /api/dashboard/themes — returns available themes + active name
- PUT /api/dashboard/theme — persists selection to config.yaml
- User custom themes discoverable from ~/.hermes/dashboard-themes/*.yaml
- Theme list endpoint added to public API paths (no auth needed)

Config:
- dashboard.theme key in DEFAULT_CONFIG (default: 'default')
- Schema override for select dropdown in config page
- Category merged into 'display' tab in config UI

i18n: theme switcher strings added for en + zh.

web/src/main.tsx

@@ -3,11 +3,14 @@ import { BrowserRouter } from "react-router-dom";
 import "./index.css";
 import App from "./App";
 import { I18nProvider } from "./i18n";
+import { ThemeProvider } from "./themes";
 
 createRoot(document.getElementById("root")!).render(
   <BrowserRouter>
     <I18nProvider>
-      <App />
+      <ThemeProvider>
+        <App />
+      </ThemeProvider>
     </I18nProvider>
   </BrowserRouter>,
 );