Add theme system with color presets, fix login page styling, persist auth session

Theme system with 5 color presets (Slate, Emerald, Violet, Amber, Rose)
and light/dark/system mode. User menu in sidebar with theme picker and
sign out. Login page uses standalone dark branded styling with autofill
override. Auth persists in sessionStorage across refreshes.

packages/admin/src/lib/api-client.ts

@@ -31,7 +31,8 @@ async function request<T>(method: string, path: string, body?: unknown): Promise
 
   if (res.status === 401) {
     useAuthStore.getState().logout()
-    window.location.href = '/login'
+    // Don't use window.location — that causes a full reload and flash
+    // The router's beforeLoad guard will redirect to /login on next navigation
     throw new ApiError('Unauthorized', 401)
   }