While editing a numeric field, the unit suffix now visually responds to
the draft content:
- Plain number / empty: suffix dims to 40% opacity (signals "default unit
if you commit a bare number")
- Explicit unit typed (hint showing): suffix fades to opacity-0 over
150ms (the conversion hint provides unit feedback instead)
- Unparseable text: suffix stays dimmed (field will revert on commit)
- At rest: unchanged full-opacity suffix
Uses opacity-0 rather than unmounting so the flex layout doesn't jump.
The predicate is simply `hint !== null` — it aligns exactly with
measurementHint's internal PLAIN_NUMBER guard.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
