fix: improve FAQ accordion accessibility

[xx7412421-cloud]

Closes #15

Summary:

• add aria-expanded, aria-controls, and stable ids to FAQ item buttons
• link answer panels with role=region and aria-labelledby
• add aria-pressed to category filters and visible focus rings
• support ArrowUp, ArrowDown, Home, and End keyboard navigation between FAQ questions

Validation:

• npm run build
• npm run lint was attempted, but this repo prompts for first-time Next.js ESLint setup instead of running non-interactively

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