fix(remote): close the /acp session cleanly on disconnect + app exit

[brettchien]

Problem

For the current build, neither closing the app nor clicking Disconnect releases the server-side /acp session:

• disconnect() only did task.abort() — the WS socket is dropped with no Close frame, and by the resume-on-reconnect design the gateway keeps the session alive for a resume that never comes.
• There was no app-teardown hook at all (.run(generate_context!()) with no RunEvent handler), so Cmd-Q / window close never even called disconnect().

Net: a quit/closed Studio leaves its session lingering on the gateway until session_ttl_hours=1 or the liveness reaper fires. With max_sessions=3, connect/quit churn can exhaust the slots.

Fix

1. Graceful disconnect — disconnect() now signals the live loop (OutMsg::Shutdown) to abandon any in-flight turn (session/cancel) and send a WS Close frame before the task ends, so the gateway sees an intentional close (not a resumable blip) and frees the slot immediately. A shared stop flag makes run_reconnecting stop instead of reconnecting; a 400 ms flush window + abort() backstop bound the teardown.
2. App-exit hook — builder switched to build() + run(|app, event| …); on RunEvent::ExitRequested it calls disconnect(), covering Cmd-Q / window close.

Verification

• rustfmt --check parses both files clean; wiring reviewed.
• Full compile is gated by CI (macOS) — this box can't build src-tauri (no GTK).
• Suggested manual check once built: connect → quit the app → confirm the gateway session count drops promptly (not after the 1 h TTL); and Disconnect button → Activity log shows a clean close with no auto-reconnect.

🤖 Generated with Claude Code