<!-- Enter a brief description/summary of your PR here. What does it
fix/what does it change/how was it tested (even manually, if necessary)?
-->
## Summary of the Pull Request
Transparency support (best-effort)
> Not every window can be made transparent. Transparency is applied on a
best-effort basis and depends on how the target app/window is built and
rendered.
## When it may not work
* Windows with special rendering pipelines (e.g., certain
hardware-accelerated / compositor-managed surfaces).
* Some tool/popup/owned windows where the foreground window isn’t the
actual surface being drawn.
## How it works (high-level)
* Resolve the best target window (preferring the top-level/root window
over transient children).
* Apply Windows’ standard layered-window alpha mechanism (per-window
opacity) to adjust transparency.
* When unpinned, Restore the original opacity/state when possible.
If transparency doesn’t change, it means the window doesn’t support this
mechanism in its current configuration.
<!-- Please review the items on the PR checklist before submitting-->
## PR Checklist
- [X] Closes:
#43278#42929#28773
<!-- - [ ] Closes: #yyy (add separate lines for additional resolved
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
https://github.com/user-attachments/assets/c97a87f2-3126-4e19-990f-8c684dbeb631
<img width="1119" height="426" alt="image"
src="https://github.com/user-attachments/assets/547671ee-81d3-4c94-8199-bf0c4b1b7760"
/>
---------
Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
