refactor(ui): move shared primitives into component layer

[beruro]

Summary

• move Placeholder, SearchSortBar, FolderHeaderRow, and list-panel layout primitives into src/components
• keep compatibility re-exports at the previous module paths
• remove two remaining upward imports from src/components
• add an executable component-boundary guard and audit record

Verification

• node scripts/quality/check-component-boundaries.mjs
• tsc --noEmit -p tsconfig.json
• ESLint on every changed TS/TSX file (0 errors)
• git diff --check

Frontend UI audit

• 1 fix applied
• 9 findings kept with documented reasons
• 0 new abstraction candidates