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/lib/api.ts

@@ -182,6 +182,16 @@ export const api = {
       },
     );
   },
+
+  // Dashboard themes
+  getThemes: () =>
+    fetchJSON<ThemeListResponse>("/api/dashboard/themes"),
+  setTheme: (name: string) =>
+    fetchJSON<{ ok: boolean; theme: string }>("/api/dashboard/theme", {
+      method: "PUT",
+      headers: { "Content-Type": "application/json" },
+      body: JSON.stringify({ name }),
+    }),
 };
 
 export interface PlatformStatus {
@@ -415,3 +425,10 @@ export interface OAuthPollResponse {
   error_message?: string | null;
   expires_at?: number | null;
 }
+
+// ── Dashboard theme types ──────────────────────────────────────────────
+
+export interface ThemeListResponse {
+  themes: Array<{ name: string; label: string; description: string }>;
+  active: string;
+}