refactor(update-account): manage every profile field with react-hook-form

• move email, phone, birthday and avatar url out of local state into the form
• validate date of birth via zod instead of the manual birthdayError flag
• derive the picker Date from the form value, dropping the state/effect sync
• prefill once so a profile refetch cannot wipe edits already in progress
• UpdateAccountFields now only takes control

Co-Authored-By: Claude Opus 5 (1M context) noreply@anthropic.com