feat(ui): native Avalonia.LayoutInspector test suite, in-app drawers, stacked chat composer & v3.16.0

[spelech]

Summary

• Native Layout Auditing: Integrates Avalonia.LayoutInspector into the headless test suite, pruning deprecated Playwright layout inspector tooling.
• In-App Slide-Out Drawers:
  • Adds Left Flank Slide-Out Drawer for Documentation and Right Flank Slide-Out Drawer for AI Assistant in \MainView.axaml.
  • Enables 100% feature availability in WebAssembly (\ISingleViewApplicationLifetime) while preserving desktop native pop-out companion window support (\DocumentationWindow, \AiAssistWindow).
  • Supports mutual exclusivity and dimmer backdrop tap-to-close.
• Modern Stacked Chat Composer:
  • Redesigns \AiAssistantTabControl.axaml\ with a full-width multiline prompt box and dedicated bottom toolbar containing Attach [📎], Model Picker [ComboBox], and Send [🚀]\ / Stop [⏹️].
  • Replaces 4 wide header buttons with a sleek [⋮]\ More Actions \MenuFlyout\ (\Setup & Endpoint, \Reload Prompts, \Clear Chat) eliminating window resize collisions.
• Responsive Layout & Touch Ergonomics:
  • Lowers \MainWindow\ \MinWidth\ from \1024\ to \768\ (and \MinHeight\ to \600) to natively support tablet viewports (\Tablet iPad).
  • Bumps tag and action button spacing to \8px\ across filter bars and settings panels to comply with touch ergonomics standards.
  • Eliminates label clipping and avoids ellipsis truncations.
• Version Bump: Bumped to \�3.16.0\ across all projects, WebAssembly bundles, documentation, and installer scripts.

Verification

• 
  pm run lint\ &
  px tsc --noEmit: Passed (0 errors).
• \dotnet test: Passed (739 passed, 0 failed, 6 live-endpoint skipped).
• \AvaloniaLayoutAuditTests: All 5 audits passed with 0 application violations across desktop (1440p, 1080p, 720p) and tablet (768px) viewports.