chore: improve screen reader support for input fields

[MerlinTheWhiz]

Closes #523

Changes made:

• Added explicit label/id, aria-describedby, aria-invalid, aria-required, helper text, and alert-linked errors in src/components/forms/FormInput.tsx:35.
• Added the same accessible wiring for auth inputs, plus labelled/stateful password visibility controls in src/app/components/auth/FormInput.tsx:40.
• Added regression tests covering labels, helper text, errors, textarea/select support, and password toggle semantics in src/components/forms/tests/FormInput.test.tsx:23 and src/app/components/auth/tests/FormInput.test.tsx:5.
• Documented usage guidance in src/app/components/accessibility/README.md:289.

Validation:

• Passed: pnpm exec vitest run src/components/forms/tests/FormInput.test.tsx src/app/components/auth/tests/FormInput.test.tsx
• Passed: Prettier check on touched files
• Blocked: pnpm run type-check fails on pre-existing syntax errors in src/components/assessment/AdaptiveTesting.tsx:43, src/components/assessment/QuestionTypes.tsx:97, and src/lib/api.ts:216.

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