[CmdPal] Animated notification

[niels9001]

Summary of the Pull Request

Gives the CmdPal toast notification a glow-up: it now slides in/out with a nice fade, has acrylic + a soft shadow, and stops fighting with SizeToContent.

Along the way, the toast guts got refactored into a couple of reusable bits that live in PowerToys.Common.UI.Controls so other PowerToys utilities can grab them for their own transient overlays:

• TransparentWindow — a WindowEx-derived host that strips the native frame, hides from taskbar/Alt-Tab, uses TransparentTintBackdrop for transparency, and runs show/hide implicit animations on its content. Supply your own animations via ShowAnimations / HideAnimations, or take the defaults (fade + slide).
• TransparentCard — a templated ContentControl with acrylic (AlwaysActiveDesktopAcrylicBackdrop), rounded corners, border, and shadow. Drop whatever XAML you want inside.
• AlwaysActiveDesktopAcrylicBackdrop — small SystemBackdrop wrapper so the acrylic doesn''t go grey when the window isn''t focused (transient overlays are never focused).

CmdPal''s ToastWindow is now basically a 16-line wrapper: derives from TransparentWindow, drops a bound TextBlock inside, and handles its own 2.5s auto-hide timer + bottom-center positioning.

cmdpal-toast.mp4

PR Checklist

• Closes: [CmdPal] Toast doesn't scale to fit content on the first time when CommandResult is KeepOpen #40886
• Communication: I''ve discussed this with core contributors already.
• Tests: Added/updated and all pass
• Localization: All end-user-facing strings can be localized
• Dev docs: Added/updated
• New binaries: Added on the required places

Detailed Description of the Pull Request / Additional comments

A couple of design notes worth calling out:

• The window is sized slightly bigger than the visible card (~24px breathing room on each side) so the shadow and slide animation have room to render without clipping. That buffer area is transparent but NOT click-through — kept it small on purpose. We explored SetWindowRgn and EnableWindow tricks to make it click-through too, but neither plays nicely with WinUI 3''s DesktopWindowXamlSource. Small transparent frame is the pragmatic compromise.
• Animations use the Toolkit''s implicit ShowAnimations / HideAnimations so there''s zero animation code in ToastWindow.xaml.cs.
• TransparentCard.xaml is registered in Themes/Generic.xaml — required for templated controls in a library project; <GenerateLibraryLayout> alone doesn''t auto-merge per-control xaml.

Validation Steps Performed

• Built clean (arm64 Debug).
• Triggered a CmdPal toast manually: fades + slides in, hangs for 2.5s, fades + slides out.
• Acrylic stays active when the window isn''t focused (toasts are never focused).
• Shadow renders fully without clipping.