browser relay: multi-session override, stale session recovery, better error feedback

- relay accepts new extension connections by closing old one (last wins, code 4000)
- extension recognizes code 4000 and stops auto-reconnect (shows "replaced" badge)
- fix ping interval race where old WS close handler killed new connection's pings
- retry CDP commands on "Session with given id not found" by re-attaching debugger
- describeError() maps known errors to user-friendly badge tooltips
- persisted relay tokens restored on server start
- user-scoped token salts, token regeneration/deletion endpoints
- extension download endpoint, integrations UI for browser relay setup
- browser relay env vars passed to pi-bridge sandboxes
- browser screen layout with chat panel and prompt prefix

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

src/server.tsx

@@ -202,7 +202,7 @@ console.log(`🚀 Server running at ${server.url}`);
 
 const BROWSER_RELAY_PORT = Number(process.env.BROWSER_RELAY_PORT ?? '18792');
 try {
-  startBrowserRelay(BROWSER_RELAY_PORT);
+  await startBrowserRelay(BROWSER_RELAY_PORT);
   console.log(`[browser-relay] listening on port ${BROWSER_RELAY_PORT}`);
 } catch (err) {
   console.error('[browser-relay] failed to start:', err instanceof Error ? err.message : err);