Prevent duplicate hotkey instances after closing Voicebox

[FelixKruger]

Summary

• register Tauri's official single-instance plugin before every other plugin
• focus the existing main window when Voicebox is launched again
• disarm the global hotkey monitor as soon as main-window shutdown starts
• exit the full Tauri app after frontend cleanup instead of closing only the main webview

Root cause

Voicebox creates the hidden Voicebox Dictate webview up front. The existing close handler only called window.close() on the main webview, so the hidden dictate window and HotkeyMonitor could keep the process alive after the user closed the app. A later manual launch then created another GUI process and another global F8/F9 listener, producing duplicate recording pills and duplicated actions.

The single-instance plugin is pinned to 2.3.7 because the current Voicebox lockfile uses Tauri 2.9.5; plugin 2.4.x requires Tauri 2.10 and would introduce an unrelated framework upgrade.

Related to #697.

Validation

• cargo check --locked
• cargo test --locked (including test_system_audio_capture)
• installed and exercised on Windows 11:
  • launching Voicebox twice left one GUI process and one backend
  • pressing F9 produced exactly one Voicebox Dictate window
  • closing the main window removed the GUI process, backend, hidden dictate window, and hotkey owner

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