[Find My Mouse] Adding transparency support for spotlight (#41701)

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fix/what does it change/how was it tested (even manually, if necessary)?
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## Summary of the Pull Request
### Feature
Separate the find my mouse's spotlight area with the backdrop, so that
we could support the frequent ask - We should leave the circle
transparent in find my mouse

### Engineering:
1. Modernize the framework - From UWP composition to WASDK composition
api

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## PR Checklist

- [x] Closes: #15512
- [ ] **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

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or any additional comments/features here -->
## Detailed Description of the Pull Request / Additional comments

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wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed
- [x] Data migration: Should nota break existing experience when upgrade
- [x] Should be able to configure the background and spotlight opacity 
- [x] Should be able to work with different settings


https://github.com/user-attachments/assets/6f311c03-fa79-41d3-94bb-589d853295f4

---------

Co-authored-by: Niels Laute <niels.laute@live.nl>

src/settings-ui/Settings.UI.Library/FindMyMouseProperties.cs

@@ -31,9 +31,6 @@ namespace Microsoft.PowerToys.Settings.UI.Library
         [JsonPropertyName("spotlight_color")]
         public StringProperty SpotlightColor { get; set; }
 
-        [JsonPropertyName("overlay_opacity")]
-        public IntProperty OverlayOpacity { get; set; }
-
         [JsonPropertyName("spotlight_radius")]
         public IntProperty SpotlightRadius { get; set; }
 
@@ -61,9 +58,8 @@ namespace Microsoft.PowerToys.Settings.UI.Library
             IncludeWinKey = new BoolProperty(false);
             ActivationShortcut = DefaultActivationShortcut;
             DoNotActivateOnGameMode = new BoolProperty(true);
-            BackgroundColor = new StringProperty("#000000");
-            SpotlightColor = new StringProperty("#FFFFFF");
-            OverlayOpacity = new IntProperty(50);
+            BackgroundColor = new StringProperty("#FF000000"); // ARGB (#AARRGGBB)
+            SpotlightColor = new StringProperty("#FFFFFFFF");
             SpotlightRadius = new IntProperty(100);
             AnimationDurationMs = new IntProperty(500);
             SpotlightInitialZoom = new IntProperty(9);