Move window titlebar logic and ui to new CustomWindowTitleBar

[DavidGBrett]

Summary by cubic

Centralized the window title bar UI and logic into a reusable CustomWindowTitleBar. All affected windows now share consistent minimize/maximize/close behavior and state handling.

Summary of changes

• Changed: Windows (SettingWindow, PluginSettingsWindow, ReleaseNotesWindow, SelectBrowserWindow, SelectFileManagerWindow, WelcomeWindow) now use CustomWindowTitleBar; SettingWindow persists state via the control’s LastNonMinimizedWindowStateChanged event.
• Added: Reusable CustomWindowTitleBar (.xaml + .xaml.cs) with IconSource and button enable/disable properties (ShowMinimizeButton, ShowMaximizeRestoreButton, ShowCloseButton); built-in minimize/maximize/restore/close; handleable button click events; host window integration (StateChanged/Activated/Closed) and an Alt+Tab restore fix.
• Removed: Per-window title bar XAML and code-behind (button handlers, state tracking, maximize/restore visibility logic, window event wiring) from the affected windows.
• Memory usage: Neutral to slightly lower due to removed duplication.
• Security risks: None; UI-only refactor with no changes to permissions or data flow.
• Unit tests: No new unit tests.

Release Note
All settings and dialog windows, including the Welcome screen, now use a consistent title bar with the same minimize, maximize, and close behavior.

^{Written for commit 779ad97. Summary will update on new commits.}