frontend: block past dates in appointment/invoice pickers; drop stale Settings Features

- Appointment date picker disables days before today.
- Invoice issue-date picker disables past days by default, with an opt-in
  "Back-date" checkbox for recording older invoices (edit mode keeps past
  dates). Added invoices.dialog.backdate to all locales.
- Removed the inert, outdated "Features" section (patient-owned storage /
  require signed toggles) from Settings and its now-unused i18n keys.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.claude/skills/heroui-migration

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+../../.agents/skills/heroui-migration

.claude/skills/heroui-native

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+../../.agents/skills/heroui-native

.claude/skills/heroui-react

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+../../.agents/skills/heroui-react