Split admin login into its own route and add auth guard to /admin

- Add /admin/login route for Login.vue as a standalone page
- Add requiresAdmin guard to /admin route
- Update auth guard redirect to /admin/login with redirect query param
- Update nginx @auth_denied to redirect to /admin/login
- Remove Login component from Admin.vue; drop v-if auth checks (guard handles access)
- Remove stale view files from old views/ structure (moved in prior commit)

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

nginx/nginx.conf.template

@@ -246,7 +246,7 @@ http {
         }
 
         location @auth_denied {
-            return 302 /admin;
+            return 302 /admin/login;
         }
 
         location /searxng {