Fix ContentDialog theme forwarding and default inheritance

[kusutoriクスとり (kusutori)]

Summary

Forward .RequestedTheme(...) to the real ContentDialog instead of only its placeholder. When unset, follow the declaration site's ActualTheme, including live changes and local/multi-window overrides.

The binding is detached on close/unmount. Explicit Default remains explicit, native .Set(...) callbacks run last, and the guide documents the behavior. No new public API.

Linked issue

Fixes #1388

Test plan

Real WinUI fixtures compare the visible dialog's theme and text foreground against a plain native ContentDialog. They cover Light/Dark/Default, both opening paths, live updates, override removal, secondary windows, draft preservation and teardown.

Targeted run	Result
Unchanged baseline + same tests	27/74 checks fail; native positive controls pass
Fixed JIT / Native AOT	74/74 checks pass in each
MSIX	All 4 theme fixtures pass (10 runner tests including guards)
Existing ContentDialogLive regressions	All 9 fixtures pass

Removing initial forwarding, update forwarding or the theme-change subscription makes the corresponding tests fail. Framework Release build passed; Tonarink integration was also verified manually.

Risk / breaking changes

Unset dialogs now follow their declaration's effective theme rather than the application fallback. Targeted validation only, not the full solution/CI suite; ARM64 and combined AOT+MSIX were not tested locally.

[Alexandre Zollinger Chohfi (azchohfi)]

kusutoriクスとり (@kusutori) A couple of conflicts with main.

[kusutoriクスとり (kusutori)]

Rebase is done and tests have passed.