Implement WM_APPCOMMAND hooks as a backup to keyboard hooks

[unchihugo]

A quick, simple implementation of WM_APPCOMMAND hooks as an alternative to existing keyboard hooks (discussed in #466). Hooks to WM_SHELLHOOK to retrieve global WM_APPCOMMAND messages. No additional permissions required. Likely fixes #437

[Copilot]

Pull request overview

Adds a WM_SHELLHOOK/HSHELL_APPCOMMAND-based path to trigger media flyouts from global WM_APPCOMMAND events, providing an alternative to the existing low-level keyboard hook approach discussed in #466 and intended to address regressions like #437.

Changes:

• Register and handle the SHELLHOOK window message to detect WM_APPCOMMAND media/volume commands and show the media flyout.
• Add Win32 constants and P/Invoke declarations for shell hook window registration/unregistration.
• Update cleanup logic to deregister the shell hook window.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
FluentFlyoutWPF/MainWindow.xaml.cs	Registers shell hook window and handles WM_SHELLHOOK/HSHELL_APPCOMMAND to show media flyouts.
FluentFlyoutWPF/Classes/NativeMethods.cs	Adds HSHELL/APPCOMMAND constants and P/Invokes for RegisterShellHookWindow/DeregisterShellHookWindow.

Comments suppressed due to low confidence (1)

FluentFlyoutWPF/MainWindow.xaml.cs:1415

• On Explorer restart (WM_TASKBARCREATED), shell hook registrations are typically lost along with other shell integrations. The recovery path currently recreates the tray icon but never re-registers the shell hook window, so WM_APPCOMMAND-based flyouts can stop working after an Explorer restart. Re-register RegisterShellHookWindow (and re-acquire WM_SHELLHOOK if needed) once WaitForExplorerReadyAsync() succeeds.

        else if (msg == WM_TASKBARCREATED)
        {
            Logger.Warn("Explorer restart detected (TaskbarCreated)");

            ExplorerRestarting = true;

            // Defer recovery, do NOT touch tray/taskbar immediately
            Dispatcher.BeginInvoke(async () =>
            {
                try
                {
                    // Wait for Explorer to actually stabilize
                    if (await WaitForExplorerReadyAsync())
                    {
                        ExplorerRestarting = false;
                        Logger.Info("Explorer stabilized, resuming taskbar integration");

                        // Now it is safe to recreate tray icon
                        RecreateTrayIconSafely();
                    }

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