fix(win/app): close every window before Application.Exit so Quit exits

[LoganRupe]

What and why

Quit from the tray panel could leave the process running with the CursorFollow Output window still up. Fixes #5.

It happens once the Settings window has been opened. Application.Exit() then closed only some of the app's three windows and returned. The window it skipped (output in one run, Settings in another) kept the WinUI message loop going. Evidence from the hung process:

• UI thread stack: idle in FrameworkApplication::RunDesktopWindowMessageLoop → GetMessageW, so nothing is blocked. The loop just never quits.
• A temporary trace of each window's Closed event: panel and Settings closed, Exit() returned, and the output window never got a close.
• Without Settings open (two windows), Exit closes everything and the process ends.

ExitApplication now closes Settings, the panel and the output window itself before calling Exit(). No forced Environment.Exit.

Not a regression from #4. The framework-dependent Debug build (dotnet build) hangs the same way.

Platforms

• macOS: built and tested
• Windows: built and tested
• Linux: built and tested
• Other platforms need the same change: no, Windows-only shutdown path

Windows testing was scripted (tray click → select target → Resume → optional Settings → Quit → confirm) with a capture running at ~30 fps:

Build	Before	After
Debug, Settings open	hung (output window left)	3/3 exited in 0.13 s
Self-contained publish / MSI, Settings open	hung (Settings window left)	3/3 exited in ≤0.15 s
MSI, Settings opened then cancelled	not tested	3/3 exited in ≤0.13 s
Debug / MSI, no Settings	exited	4/4 exited in ≤0.13 s

dotnet test tests/CursorFollowWin.Tests: 20/20 passed.

Checklist

• CHANGELOG.md updated under Unreleased (if user-visible)
• Signing, entitlements or identifiers changed? Called out above.