Port ViewExtensions from 7.x

**Source**: Microsoft.Toolkit.Uwp.UI (7.x)
**Destination**: CommunityToolkit.WinUI.Extensions (8.x) — not yet ported

## Description

The ApplicationViewExtensions and TitleBarExtensions types provide a declarative way of setting ApplicationView, CoreApplicationView and ApplicationViewTitleBar properties from XAML.

## Prerequisites

None identified.

## Porting Basics

- [ ] Namespace updated to `CommunityToolkit.WinUI.*`
- [ ] Code for UWP, WinUI3 and Uno Platform consolidated
- [ ] Desired MultiTargets configured
- [ ] Sample code migrated
- [ ] Concept docs migrated
- [ ] Tests migrated and passing

## Quality Evaluation

See [Porting Criteria](https://github.com/CommunityToolkit/Windows/wiki/Component-Quality-Evaluation) for detailed evaluation guidance.

- [n/a] UX: Accessibility reviewed
- [ ] UX: Visual design reviewed
- [n/a] UX: Interaction patterns appropriate
- [ ] DX: API surface reviewed
- [ ] DX: API flexibility appropriate
- [ ] DX: Documentation updated
- [ ] MX: Breaking changes documented
- [ ] MX: Domain isolation appropriate
- [ ] MX: Test coverage adequate

## Tracking Gaps

*No gaps identified — porting work not yet started.*

## Closing

- [ ] All Porting Basics complete
- [ ] Quality Evaluation reviewed
- [ ] Gaps filed as sub-issues

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## 7x Documentation Inventory

**Docs ported to 8x**: n/a

**Docs not in 8x**: All sections

## 7x Source Inventory

**Source already ported to 8x**: n/a

**Source not ported to 8x**: n/a

## Notes

- Not currently planned for porting. Open to community interest.
- **Migration option**: The TitleBar component in Labs should be considered as an alternative.
- **Archive redirect**: https://learn.microsoft.com/en-us/dotnet/communitytoolkit/windows/archive/viewextensions

Beginne mit der Durchsicht der archivierten ViewExtensions-Dokumentation und der TitleBar-Komponente in Labs als genannter Migrationsalternative. Verwende die Porting Criteria, um die Arbeiten an Namespace, Ziel, Beispiel, Dokumentation und Tests zu planen; abgeschlossen bedeutet, dass die Checklisten Porting Basics und Quality Evaluation vollständig abgearbeitet sind und alle Tests bestehen.

Vom Indexierungsmodell aus dem Issue-Text verfasst.