CmdPal: Transparent window (#45159)

## Summary

This PR adds:
- Backdrop material customization
  - Alongside acrylic, the following options are now available:
  - Transparent background
  - Mica background
- Background material opacity
  - Lets you control how transparent the background is

## Pictures? Pictures!

<img width="1491" height="928" alt="image"
src="https://github.com/user-attachments/assets/ff4e9e06-fcf1-4f05-bc0a-fb70dc4f39be"
/>



https://github.com/user-attachments/assets/84e83279-afab-481e-b904-f054318c5d2f

<img width="977" height="628" alt="image"
src="https://github.com/user-attachments/assets/241a228d-af3f-448a-94a6-0a282218bd8c"
/>


## PR Checklist

- [x] Closes: #44197
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issues) -->
- [ ] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [ ] **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
- [ ] [JSON for
signing](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ESRPSigning_core.json)
for new binaries
- [ ] [WXS for
installer](https://github.com/microsoft/PowerToys/blob/main/installer/PowerToysSetup/Product.wxs)
for new binaries and localization folder
- [ ] [YML for CI
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ci/templates/build-powertoys-steps.yml)
for new test projects
- [ ] [YML for signed
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/release.yml)
- [ ] **Documentation updated:** If checked, please file a pull request
on [our docs
repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here: #xxx

<!-- Provide a more detailed description of the PR, other things fixed,
or any additional comments/features here -->
## Detailed Description of the Pull Request / Additional comments

<!-- Describe how you validated the behavior. Add automated tests
wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed

src/modules/cmdpal/Microsoft.CmdPal.UI.ViewModels/BackdropStyles.cs

@@ -0,0 +1,65 @@
+// Copyright (c) Microsoft Corporation
+// The Microsoft Corporation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+namespace Microsoft.CmdPal.UI.ViewModels;
+
+/// <summary>
+/// Central registry of backdrop style configurations.
+/// </summary>
+public static class BackdropStyles
+{
+    private static readonly Dictionary<BackdropStyle, BackdropStyleConfig> Configs = new()
+    {
+        [BackdropStyle.Acrylic] = new()
+        {
+            ControllerKind = BackdropControllerKind.Acrylic,
+            BaseTintOpacity = 0.5f,
+            BaseLuminosityOpacity = 0.9f,
+            PreviewBrush = PreviewBrushKind.Acrylic,
+        },
+        [BackdropStyle.AcrylicThin] = new()
+        {
+            ControllerKind = BackdropControllerKind.AcrylicThin,
+            BaseTintOpacity = 0.0f,
+            BaseLuminosityOpacity = 0.85f,
+            PreviewBrush = PreviewBrushKind.Acrylic,
+        },
+        [BackdropStyle.Mica] = new()
+        {
+            ControllerKind = BackdropControllerKind.Mica,
+            BaseTintOpacity = 0.0f,
+            BaseLuminosityOpacity = 1.0f,
+            PreviewBrush = PreviewBrushKind.Solid,
+            FixedOpacity = 0.96f,
+            SupportsOpacity = false,
+        },
+        [BackdropStyle.MicaAlt] = new()
+        {
+            ControllerKind = BackdropControllerKind.MicaAlt,
+            BaseTintOpacity = 0.0f,
+            BaseLuminosityOpacity = 1.0f,
+            PreviewBrush = PreviewBrushKind.Solid,
+            FixedOpacity = 0.98f,
+            SupportsOpacity = false,
+        },
+        [BackdropStyle.Clear] = new()
+        {
+            ControllerKind = BackdropControllerKind.Solid,
+            BaseTintOpacity = 1.0f,
+            BaseLuminosityOpacity = 1.0f,
+            PreviewBrush = PreviewBrushKind.Solid,
+        },
+    };
+
+    /// <summary>
+    /// Gets the configuration for the specified backdrop style.
+    /// </summary>
+    public static BackdropStyleConfig Get(BackdropStyle style) =>
+        Configs.TryGetValue(style, out var config) ? config : Configs[BackdropStyle.Acrylic];
+
+    /// <summary>
+    /// Gets all registered backdrop styles.
+    /// </summary>
+    public static IEnumerable<BackdropStyle> All => Configs.Keys;
+}