Harden app shutdown so Exit fully terminates process

[zortos293]

This PR hardens the Electron app shutdown path so selecting Exit reliably terminates the process instead of leaving it running headless. Previously, calling app.quit() from the QUIT_APP IPC handler relied on the normal window-close path while long-lived timers, websockets, and Discord RPC could keep the process alive indefinitely.

Shutdown orchestration (src/main/index.ts):

• Add runShutdownCleanup() to idempotently stop refreshScheduler, disconnect signalingClient, dispose appUpdater, destroy mainWindow, and tear down Discord RPC
• Add requestAppShutdown() to coordinate explicit quit requests, running cleanup before app.quit() and optionally scheduling a 2s fallback to app.exit(0)
• Route IPC_CHANNELS.QUIT_APP through requestAppShutdown() with force-exit fallback enabled
• Reuse centralized cleanup in before-quit instead of duplicating logic
• Add isShutdownRequested/isShutdownCleanupComplete guards and will-quit/quit handlers to cancel fallback timers on normal termination
• Block activate recreation if shutdown is already requested

Updater coordination (src/main/index.ts, src/main/updater.ts):

• Add onBeforeQuitAndInstall/onQuitAndInstallError hooks to AppUpdaterControllerOptions
• Call hooks in quitAndInstall() to set/clear isUpdaterInstallQuitInProgress flag
• Suppress force-exit fallback during updater-driven install/restart to avoid disrupting updater.quitAndInstall()

Timer hardening (src/main/updater.ts, src/main/services/refreshScheduler.ts, src/main/gfn/signaling.ts):

• Call .unref() on startupTimer, intervalTimer, refreshTimer, and heartbeatTimer so they do not keep the process alive on their own