Fix TabPanel: tab strip hidden by content and ignores h_alignment (#86)

[emosaru]

Summary

Fixes two bugs in the TabControl template in LayoutControl.axaml, both reported in issue #86:

• Tab strip covered by content — The global ClipToBounds=False style allows child content (e.g. a scroll layout) to overflow upward out of its grid row, visually hiding the tab header strip above it. Fixed by setting ZIndex="1" on the ItemsPresenter so the tab strip always renders on top of any overflowing content.

• h_alignment ignored — The ItemsPresenter had HorizontalAlignment hardcoded to Center, so the layout's h_alignment JSON field had no effect on the tab strip. Fixed by binding HorizontalAlignment to the TabPanel data model via TrivialEnumConverter, matching how every other LayoutItem applies the same property.

Note on wrapped rows: when tabs overflow to multiple lines, WrapPanel does not support per-row alignment — each wrapped row will still align to the left. This is a WrapPanel limitation and is not addressed here.

Test plan

• Open the test pack from the issue (ALBWR tracker) and confirm the world map scroll layout no longer covers the tab headers
• Set h_alignment to left, center, and right on a tabbed layout and confirm the tab strip aligns accordingly on a single row
• Confirm no visual regressions on other packs with tabbed layouts

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