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/App.tsx

@@ -9,6 +9,7 @@ import AnalyticsPage from "@/pages/AnalyticsPage";
 import CronPage from "@/pages/CronPage";
 import SkillsPage from "@/pages/SkillsPage";
 import { LanguageSwitcher } from "@/components/LanguageSwitcher";
+import { ThemeSwitcher } from "@/components/ThemeSwitcher";
 import { useI18n } from "@/i18n";
 
 const NAV_ITEMS = [
@@ -67,6 +68,7 @@ export default function App() {
           </nav>
 
           <div className="ml-auto flex items-center gap-2 px-2 sm:px-4">
+            <ThemeSwitcher />
             <LanguageSwitcher />
             <span className="hidden sm:inline font-display text-[0.7rem] tracking-[0.15em] uppercase opacity-50">
               {t.app.webUi}