feat(system-ui): refine XP theme and landing

[doodlewind]

Summary

• replace the public landing page with a centered Pocket Desktop mark and Coming Soon, and restore the mark lens as a smooth solid circle
• move XP font, semantic icon, Start menu, window chrome, taskbar, and Explorer presentation behind the theme boundary
• add an XP two-column Start menu and Explorer toolbar, address bar, task pane, details columns, and status bar while preserving Classic behavior
• add original Luna-inspired SVG assets plus reproducible Inter font atlases with OFL attribution

Validation

• bun run check
• bun run capture
• bun run build
• bun run build:site
• bun run test:web
• bun run test:site

[doodlewind]

Superseded by #8, which rebuilds the XP chrome from Luna measurements on the merged baseline. Continuing there.