fix(a11y): complete file tab semantics

[frobel0520]

Summary

• add stable tab and tabpanel IDs for Package, Build and REST Labs
• add aria-controls, aria-labelledby, roving tabIndex and horizontal orientation
• support Arrow, Home and End keyboard navigation with focus movement
• add shared navigation helper coverage

Verification

• npm --prefix frontend test -- src/components/tab-navigation.test.ts src/topics/package/lab.test.ts src/topics/build/lab.test.ts src/topics/rest/lab.test.ts src/topics/package/content.test.ts src/topics/build/content.test.ts src/topics/rest/content.test.ts
• npm --prefix frontend run lint