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>

frontend/src/routes/admin/+page.svelte

@@ -47,7 +47,7 @@
 		const token = getToken();
 		const role = getRole();
 		if (!token || role !== 'admin') {
-			goto('/join');
+			goto('/admin/login');
 			return;
 		}
 		await reload();