[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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## 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/modules/MouseUtils/FindMyMouse/FindMyMouse.h

@@ -11,9 +11,9 @@ enum struct FindMyMouseActivationMethod : int
 };
 
 constexpr bool FIND_MY_MOUSE_DEFAULT_DO_NOT_ACTIVATE_ON_GAME_MODE = true;
+// Default colors now include full alpha. Opacity is encoded directly in color alpha (legacy overlay_opacity migrated into A channel)
 const winrt::Windows::UI::Color FIND_MY_MOUSE_DEFAULT_BACKGROUND_COLOR = winrt::Windows::UI::ColorHelper::FromArgb(255, 0, 0, 0);
 const winrt::Windows::UI::Color FIND_MY_MOUSE_DEFAULT_SPOTLIGHT_COLOR = winrt::Windows::UI::ColorHelper::FromArgb(255, 255, 255, 255);
-constexpr int FIND_MY_MOUSE_DEFAULT_OVERLAY_OPACITY = 50;
 constexpr int FIND_MY_MOUSE_DEFAULT_SPOTLIGHT_RADIUS = 100;
 constexpr int FIND_MY_MOUSE_DEFAULT_ANIMATION_DURATION_MS = 500;
 constexpr int FIND_MY_MOUSE_DEFAULT_SPOTLIGHT_INITIAL_ZOOM = 9;
@@ -30,7 +30,6 @@ struct FindMyMouseSettings
     bool doNotActivateOnGameMode = FIND_MY_MOUSE_DEFAULT_DO_NOT_ACTIVATE_ON_GAME_MODE;
     winrt::Windows::UI::Color backgroundColor = FIND_MY_MOUSE_DEFAULT_BACKGROUND_COLOR;
     winrt::Windows::UI::Color spotlightColor = FIND_MY_MOUSE_DEFAULT_SPOTLIGHT_COLOR;
-    int overlayOpacity = FIND_MY_MOUSE_DEFAULT_OVERLAY_OPACITY;
     int spotlightRadius = FIND_MY_MOUSE_DEFAULT_SPOTLIGHT_RADIUS;
     int animationDurationMs = FIND_MY_MOUSE_DEFAULT_ANIMATION_DURATION_MS;
     int spotlightInitialZoom = FIND_MY_MOUSE_DEFAULT_SPOTLIGHT_INITIAL_ZOOM;