feat: add /admin/login page (v0.14.0)

Password form at /admin/login that calls POST /api/v1/admin/login and
redirects to /admin on success. Admin dashboard now redirects to
/admin/login instead of /join when unauthenticated. Test guide updated.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

TEST_GUIDE.md

@@ -60,23 +60,7 @@ Please test each step in order and report any errors (console errors, wrong text
 
 ## Admin & Host Features
 
-For these steps you need an admin session. Log in via the admin API (or use the curl command below):
-
-```bash
-curl -s -X POST http://localhost:3000/api/v1/admin/login \
-  -H 'Content-Type: application/json' \
-  -d '{"password":"admin123"}' | python3 -m json.tool
-```
-
-Copy the `jwt` value — you'll need it for the Authorization header in curl commands.
-To use the admin dashboard in the browser, paste the token into DevTools console:
-
-```js
-localStorage.setItem('token', '<jwt>');
-localStorage.setItem('role', 'admin');
-```
-
-Then navigate to **http://localhost:5173/admin**.
+For these steps you need an admin session. Go to **http://localhost:5173/admin/login** and enter the admin password (`admin123` for the dev environment). You'll be redirected to the dashboard automatically.
 
 ### Step 10 — Admin Dashboard: Stats & Config
 1. Go to **http://localhost:5173/admin**