fix(app): force SSE reconnect on tab refocus after long hidden duration - #19461

Summary

• Fix web UI not refreshing data when the tab is refocused after being hidden
• Track when the tab becomes hidden and force SSE stream reconnect if either the event stream is stale or the tab was hidden longer than the heartbeat timeout (15s)

Problem

The onVisibility handler in global-sdk.tsx only checked Date.now() - lastEventAt < HEARTBEAT_TIMEOUT_MS when the tab regained focus. This missed the common scenario where:

1. A heartbeat arrives at T=0
2. User switches tabs at T=1s
3. Browser freezes the tab's JS timers
4. User returns at T=12s
5. Date.now() - lastEventAt = 12s < 15s — handler does nothing
6. The SSE connection may have silently died, but the client doesn't reconnect

Fix

Track hiddenAt on visibilitychange: hidden, then on visible force reconnect if either condition is true:

• No SSE event received in 15s (original check)
• Tab was hidden for 15s+ (new check — catches frozen-timer cases)

The downstream refresh flow (server.connected → queue.refresh() → bootstrap() + child directory re-bootstrap) already handles full state sync once the SSE reconnects. No other files need changes.

Testing

• All 299 existing unit tests pass
• Typecheck passes