Windows: Add close-to-tray support (keep running in background when console closes)

[edmonbpb]

Summary

Adds "close-to-tray" functionality for the Windows build. When the user closes the console window (clicking the X button), Aether detaches from the console and continues running the tunnel in the background. A system tray icon appears with an "Exit" menu item for clean shutdown.

How it works

• Registers a console Ctrl handler via exttt{SetConsoleCtrlHandler} that intercepts exttt{CTRL_CLOSE_EVENT}, exttt{CTRL_LOGOFF_EVENT}, and exttt{CTRL_SHUTDOWN_EVENT}.
• On these events, calls exttt{FreeConsole()} to detach from the console, preventing the ~5s force-termination.
• Creates a system tray icon using the exttt{tray-icon} crate (with exttt{common-controls-v6} for proper Windows 10/11 menu rendering) that provides an "Exit" item to cleanly terminate the process.

Platform & dependencies

• Windows-only ( exttt{[cfg(target_os = "windows")]}).
• New Windows-only dependencies under exttt{[target.'cfg(windows)'.dependencies]}:

  • exttt{tray-icon = "0.24"} (with 	exttt{common-controls-v6} feature)
    

  • No new cross-platform dependencies.

Configuration

• Enabled by default on Windows.
• Disable with environment variable exttt{AETHER_CLOSE_TO_TRAY=0} (or exttt{false}, exttt{off}, exttt{no}).

Testing

• Verified compilation on Windows with the MSVC toolchain ( exttt{cargo check} passes for the new tray module).
• The GitHub Actions CI ( exttt{release.yml}) will run the full Windows build on exttt{windows-latest} runners to verify the complete build.

Notes

• The console handler explicitly does NOT handle exttt{CTRL_C_EVENT} or exttt{CTRL_BREAK_EVENT}, preserving the default Ctrl+C behavior (immediate exit).
• The exttt{Cargo.lock} has been updated with the new exttt{tray-icon} dependency and its transitive dependencies.

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Fixes the issue of Aether exiting completely when the console window is closed on Windows.