Add sticky header support and modernize MainWindow UI infrastructure and safer elevation handling

[aurelm-w]

This PR introduces sticky header infrastructure and refactors for the MainWindow to improve maintainability, UX and robustness.
It also improved the elevation flow to be more robust. There are also some bug fixes and refactors to improve code stability and reliability.

• MainWindow improvements and sticky header overlay support in MainWindow.xaml
• Refactored the top application bar layout with a Grid instead of manual positioning
• Added StickyHeaderSupport and StickyHeader properties
• Sticky header support and reusable sticky headers across pages
• Added IStickyHeaderSupport integration to NavItem
• Reorganized the content area to support sticky header overlays
• Elevation handling : hardened administrator detection with exception handling and improved UAC elevation flow (fix App crashes when canceling elevation request during settings access #1 )
• Improved responsiveness and removed UI positioning workarounds
• Refactored code for stability and optimization

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

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🟢 Metrics 5 complexity · 0 duplication

Metric	Results
Complexity	5
Duplication	0

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