fix(contacts): seal managed calendars off from user edits

Managed special-dates calendars were only recognised by an id cached in
preferences, and were offered as ordinary write targets — so a user could
create an event in 'Birthdays' (which the next sync then deleted), and after
a backup restore wiped the prefs the editor lock silently disappeared.

CalendarSource now carries isManaged, read from the durable CAL_SYNC2 marker
the data layer already stamps, so identity survives a restore. With it:
- the new-event calendar picker excludes managed calendars (no user events
  land in a calendar the sync owns);
- the editor lock keys off the marker, not a stored id;
- a managed event's save is forced to the whole series instead of parking in
  the scope dialog (a 'this/following' split creates rows the sync reverts);
- the detail sheet hides Delete for managed events (the sync would just
  resurrect them — a contact date is removed at its source).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

app/src/main/java/de/jeanlucmakiola/calendula/domain/Models.kt

@@ -26,6 +26,14 @@ data class CalendarSource(
      * owns for its own calendars). Always null for synced calendars.
      */
     val description: String? = null,
+    /**
+     * A special-dates mirror calendar the app manages (birthdays/anniversaries
+     * from contacts). Its events' title/date/recurrence are owned by the sync,
+     * so it's hidden from the new-event calendar picker and its events lock those
+     * fields in the editor. Recognised by a durable provider marker, so it holds
+     * even after a backup restore clears the app's stored ids.
+     */
+    val isManaged: Boolean = false,
 )
 
 data class EventInstance(