fix(a11y): add missing ARIA props across codebase

[crippledgeek]

Summary

• Pipe TanStack Form validation errors into React Aria's errorMessage prop on SelectField and TextAreaField, enabling automatic aria-describedby wiring via React Aria's built-in FieldError
• Remove unused FieldErrorMessage component (replaced by React Aria's native error handling)
• Add aria-live="polite" region around converter output so screen readers announce results
• Add aria-label to search input, aria-pressed to category filter buttons, aria-hidden to decorative icons and emojis on Home page
• Add role="status" and aria-label to all Loader variants, aria-hidden to spinner icons

Test plan

• Build passes (npm run build)
• Lint passes (npm run lint)
• Verify screen reader announces converter output after conversion
• Verify form validation errors are associated with inputs (check aria-describedby in DevTools)
• Verify category filter buttons report pressed state to assistive technology
• Verify loader components announce "Loading" status to screen readers

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