Fix backup loop, phone count invariant, and auto-start behaviour

- LocalPhotoIndex: add count(filter:) so fast-path reconcile uses the
  filtered asset count as phoneTotal instead of the unfiltered totalCount;
  fixes the broken alreadySafe + needBackup == phoneTotal invariant

- BackupStatusService: fast-path uses count(filter:) instead of totalCount;
  writeManifest falls back to NASManifestCache when NAS download fails so
  accumulated history is never silently discarded on a flaky connection

- ConnectionStore: autoBackupEnabled defaults false; new autoBackupOnOpen
  (default false) separates LAN-join auto-backup from app-open auto-backup

- AutoBackupCoordinator: onActive(lanTriggered:) tracks trigger source;
  handleReconciliationComplete gates on autoBackupEnabled for LAN joins and
  autoBackupOnOpen for app-open — prevents backup starting on every launch

- SettingsView: split auto-backup into two toggles with accurate descriptions

Co-Authored-By: Kutesir <kutesir@provoc.ug>
Co-Authored-By: Sentry <sentry@provoc.ug>

Services/LocalPhotoIndex.swift

@@ -50,6 +50,10 @@ actor LocalPhotoIndex {
 
     var totalCount: Int { records.count }
 
+    func count(filter: BackupFilter) -> Int {
+        records.values.filter { passes($0, filter: filter) }.count
+    }
+
     func allRecords() -> [Record] { Array(records.values) }
 
     func countSafe(against index: ManifestIndex, filter: BackupFilter) -> Int {