user-scoped channel sessions, whatsapp disconnect cleanup, browser relay token fix

Channel session IDs now include userId (channel-{provider}-{userId}-{contextId})
to prevent cross-user contamination in multi-user setups. WhatsApp disconnect
properly logs out and clears cached auth. Browser relay uses server-derived token
directly.

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

src/extensions/browser-relay/background-utils.js

@@ -38,8 +38,8 @@ export async function buildRelayWsUrl(host, port, gatewayToken) {
       "Missing relay token in extension settings (chrome.storage.local.relayToken)",
     );
   }
-  const relayToken = await deriveRelayToken(token, port);
-  return `ws://${host}:${port}/extension?token=${encodeURIComponent(relayToken)}`;
+  // Token is already derived server-side — use it directly
+  return `ws://${host}:${port}/extension?token=${encodeURIComponent(token)}`;
 }
 
 export function isRetryableReconnectError(err) {