fix(settings): make personalisation autosave lossless around "Other" boxes

WHY THIS MATTERS
Two silent data-loss paths were replicated live during the PR #365
review of the new Personalisation settings page:

1. Tick "Other" under Practice areas and leave the text box empty,
   then change any OTHER field (e.g. Title -> Partner). Nothing is
   saved - no request fires, no status appears, and navigating away
   discards the change. The page-wide validationError guard paused
   the entire autosave effect, and the error string itself was never
   rendered, so the user had no way to know saving had stopped.

2. Pick a value (or finish typing) and click another Settings tab
   within the 400 ms debounce window. The effect cleanup cancelled
   the scheduled save on unmount, so the edit vanished.

WHAT IS A DEBOUNCED AUTOSAVE EFFECT
Each edit re-runs a React effect that schedules the PATCH 400 ms out;
another edit inside that window cancels and reschedules. The cleanup
that makes the rescheduling work is the same cleanup that runs on
unmount - so without an explicit flush, leaving the page always
cancels the last pending save.

HOW THE FIX WORKS
- The hook now reports WHICH field groups are mid-edit-invalid
  (invalidGroups) instead of a single page-wide error. The autosave
  effect only skips a save when the edited field's own group is
  invalid; for other fields it saves a payload in which the invalid
  groups fall back to their last persisted values, so a half-finished
  "Other" box can neither block unrelated saves nor overwrite its own
  stored value with a transient empty state.
- The validation message is now rendered inline (aria-live), so a
  paused save is visible instead of silent.
- The scheduled save closure is kept in a ref; the unmount cleanup
  fires it immediately instead of dropping it.

Tests pin all three behaviours: unrelated-field saves proceed with the
persisted fallback, the message renders, and unmount flushes the
pending save.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RuPCULDYgVsiCanRgbEy6W