docs(contacts): translations, store copy + changelog for special dates

- German translations for the special-dates sub-page, the managed-field editor
  hint, and the calendar/title strings.
- Store listing (en/de): a paragraph on the optional, on-device, one-way contact
  special-dates feature.
- Soften the calendar-permission rationale ("all it asks for up front") now that
  an optional contacts permission exists, without weakening the privacy promise.
- CHANGELOG entry for #15.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

fastlane/metadata/android/en-US/full_description.txt

@@ -8,6 +8,11 @@ changes (only this event / this and all following / the whole series) and a
 simple repeat picker. Calendula also delivers your event reminders as
 notifications — tap one and you're on the event.
 
+Optionally, show your contacts' birthdays, anniversaries and other special
+dates in your calendar. You turn this on yourself; contacts are read only on
+your device, are never uploaded, and are never changed — Calendula only
+mirrors the dates into local calendars you fully control.
+
 The differentiator is the design: real Material 3 Expressive throughout,
 with dynamic color, expressive motion, and expressive shapes.