[Common][PowerDisplay][QuickAccess] Shared flyout positioning helper

[niels9001]

Summary

Introduces a shared FlyoutWindowHelper in Common.UI.Controls and migrates both PowerDisplay and QuickAccess to it, eliminating two pre-existing flyout positioning bugs and removing duplicated math.

Bugs fixed

1. PowerDisplay flyout overlapped the taskbar at 100% scaling

The previous PowerDisplay-only positioning math anchored to the screen bounds rather than DisplayArea.WorkArea on certain monitor configurations, so the bottom edge of the flyout could land on top of the taskbar.

2. QuickAccess flyout rendered too large / partially off-screen after switching DPI between 150% and 100%

The previous code passed WindowEx.Width/Height into MoveAndResize on every summon. Those properties are not the XAML literals — they are computed live as AppWindow.Size / GetDpiForWindow() * 96. After a system-scaling switch, the runtime size has drifted, that wrong "DIP" value got fed into MoveAndResize, and the destination DPI multiplier scaled it again → wrong size, and the wrong size shifted the bottom-right anchor off-screen.

QuickAccess now caches the XAML design size once at construction (when the values are still trustworthy) and uses the cache as the source of truth.

How the helper works

• Uses absolute screen coordinates against DisplayArea.WorkArea, so it handles non-primary and negatively-positioned monitors correctly.
• Performs a 1×1 MoveAndResize "teleport" onto the target display before the final visible-size call. The 1×1 jump may cross a DPI boundary, but it's invisible; the second call sets the real size while the window is already on the destination monitor, so no DPI boundary is crossed for the rendered size and WM_DPICHANGED never fires on a visible window.
• Exposes overloads for bottom-right anchoring (both flyouts) and centered placement (PowerDisplay's IdentifyWindow).

This teleport-then-size approach matches the technique the original Settings.UI flyout used for years before it was removed.

Cleanup

• Deletes the PowerDisplay-only DpiSuppressor — its WM_DPICHANGED-suppression code path is now dead because the helper sidesteps the message entirely.
• The DpiSuppressor class also doubled as a generic WndProc subclass to route WM_HOTKEY into HotkeyService. That piece is preserved as WindowMessageHook in Common.UI.Controls/Window/ since PowerDisplay still needs in-process hotkey handling.

Validation

• Common.UI.Controls, PowerDisplay, and QuickAccess build clean (x64/Debug).
• Manual repro:
  • PowerDisplay flyout no longer overlaps taskbar at 100% scaling, on multiple invocations.
  • QuickAccess renders at the correct size and position when switching system scaling between 150% and 100%.
  • PowerDisplay hotkey toggle still works after the DpiSuppressor → WindowMessageHook rename.

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[moooyo]

LGTM, tested on my local env.