fix: close SSH tunnels on window close and app termination

[datlechin]

Summary

Closes #214 — SSH tunnel processes (ssh -N -L) were not terminated when closing a connection window or quitting the app.

Four root causes fixed:

• Unreliable disconnect trigger: The only path to disconnectSession() was SwiftUI's onDisappear, which can miss window closes. Now WindowLifecycleMonitor posts .lastWindowDidClose via NSWindow.willCloseNotification (always fires), and DatabaseManager observes it as a deterministic disconnect path.
• No cleanup on app termination: applicationWillTerminate now calls SSHTunnelManager.terminateAllProcessesSync() to kill all SSH processes before exit.
• waitForProcessExit could hang forever: If the SSH process exited between terminate() and setting terminationHandler, the continuation never resumed. Now guards isRunning, races termination handler vs timeout via TaskGroup, and SIGKILLs as fallback.
• closeAllTunnels() didn't wait: Now waits for all processes concurrently via TaskGroup.

Test plan

• Connect to a database via SSH tunnel
• Run lsof -i | grep ssh — confirm tunnel process exists
• Close the connection window
• Run lsof -i | grep ssh — confirm tunnel process is gone
• Test with multiple windows for same connection (only last close should disconnect)
• Test app quit with active SSH tunnel — confirm process is terminated

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