frontend: add admin panel for user and permission management

Admin-only page at /admin with:
- Create user form (username, password, admin toggle)
- User list table with edit and delete actions
- Inline user editing (change password, toggle admin role)
- Inline permission editor with per-app checkboxes grouped by server
- Access guarded by checking is_admin from /api/auth/me

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

gamestream-webtransport-proxy/src/proxy/mod.rs

@@ -85,9 +85,8 @@ pub async fn validate_stream_token(proxy: &Proxy, provided: &str) -> std::result
     match token_guard.take() {
         Some(expected) if expected == provided => Ok(()),
         Some(_) => {
-            // Wrong token: still consumed by the `take()` above. Any validation
-            // attempt — correct or not — invalidates the token, so a wrong
-            // guess cannot be followed by a correct one.
+            // Put the token back since it wasn't matched
+            // Actually no — the design is that any attempt consumes it for security
             Err("Invalid stream token".to_string())
         }
         None => Err("Stream token already used".to_string()),