fix: broaden restore availability and stabilize the export picker

- Restore is import, not export: offer it whenever any writable, non-
  managed calendar exists (local or synced), not only when there is a
  local calendar to back up. Previously the row lived inside the
  export-gated block and vanished for users with only a writable synced
  calendar, despite import supporting that target.
- Export-picker selection now uses rememberSaveable and is no longer
  keyed on the observer-driven calendars list, so a background provider
  re-emit (sync/recolor) can't silently reset the user's de-selections,
  and the choice survives rotation.
- Shared calendar picker: restore the displayName fallback for a synced
  calendar whose account name and type are both blank (was grouping them
  under an empty header).
- Drop imports left dead by the CalendarPickerGroups extraction.

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

app/src/main/res/values/strings.xml

@@ -431,6 +431,7 @@
     <string name="calendars_export_title">Export calendars</string>
     <string name="calendars_export_hint">Choose which calendars to include in the .ics file.</string>
     <string name="calendars_export_action">Export</string>
+    <string name="calendars_restore_header">Restore</string>
     <string name="calendars_restore_action">Restore from .ics file</string>
     <string name="calendars_restore_hint">Import events from a backup or another calendar app.</string>
     <string name="calendars_auto_backup">Automatic backup</string>