add CSS migration foundation and screenshot tooling (Phase 0)

- CSS file structure with @layer declaration (reset, layout, components, utilities, overrides)
- Layout primitives: .stack, .cluster, .row, .auto-grid, .container-page, .with-sidebar, .center
- mix screenshots task using Playwright for visual regression testing
- Golden baseline captured (10 pages x 4 breakpoints = 40 screenshots)
- No visual changes — new CSS not wired into any layout yet

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -73,6 +73,9 @@ npm-debug.log
 # API reference specs (development only)
 /docs/api-specs/
 
+# Visual regression screenshots (generated by mix screenshots)
+/screenshots/
+
 # Database files
 *.db
 *.db-*