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/apps/officer-web/Screens/Dashboard/Browser/BrowserScreen.tsx

@@ -8,6 +8,8 @@ import { defaultLayout } from './defaultLayout';
 import { TabList } from './TabList';
 import { TabPreview } from './TabPreview';
 
+const PROMPT_PREFIX = `You are a browser assistant. The user has connected Chrome tabs via the Officer Browser Relay. You have a 'browser' tool — use it to list tabs, take screenshots, navigate, evaluate JavaScript, and get page info. When asked about a page, take a screenshot first.`;
+
 export const BrowserScreen = () => {
   const isMobile = useIsMobile();
   const [selectedId, setSelectedId] = useGlobal<string | null>('BROWSER_SELECTED_TAB', null);
@@ -30,6 +32,7 @@ export const BrowserScreen = () => {
         workspace={workspace}
         locked
         components={components}
+        promptPrefix={PROMPT_PREFIX}
         mobilePanelId={mobilePanelId}
         onMobilePanelChange={mobilePanelId ? () => onMobileBack() : undefined}
       />