fix(ui): replace emojis with vector icons and flatten studio engine subnav

[spelech]

Summary

1. Vector Icons for Web & Desktop: Avalonia Browser-WASM with Skia lacks emoji glyphs in the Inter font, causing unicode emojis to render as four stacked horizontal bars (.notdef tofu box). Added StreamGeometry vector icons to DesignTokens.axaml and replaced all TextBlock emoji symbols across controls with vector PathIcon elements.
2. Flatten Studio Engine Subnav: Replaced the two-level navigation with direct flat workflow buttons aligned on the left of DynamicStageContainerControl.axaml: Images, Text, Video, 3D Mesh, Audio, and Sticker Studio.
3. Removed ComboBox Dropdown: Removed the redundant modality ComboBox dropdown from EngineStudioTabControl.axaml in favor of the direct top-bar workflow buttons.
4. Updated Default Startup Domain: Set default startup domain to Installed Models (SelectedTabIndex = 0 and NavigationRail.SelectedDomain = NavDomain.Models). Default studio workflow mode set to Images with IsStickerStudioActive = false.
5. Fresh Playwright Screenshots & WASM Bundle: Recompiled WebAssembly distribution with vector icons and re-ran PlaywrightScreenshotGenerator to refresh all documentation PNGs and added dashboard_sticker_studio.png.

Verification

• npm run lint passed cleanly.
• npx tsc --noEmit passed cleanly.
• dotnet test passed all unit, layout audit, and headless interaction tests.
• Freshly generated Playwright screenshots verified.