fix: prune orphaned folder rows alongside photo rows

prune_missing_photos() previously only deleted Photo rows whose files
were gone, leaving every folder row from the old library in the DB —
which made the sidebar tree wildly out of sync with the on-disk
structure (still showing /photos/2024/, /photos/2026/03/, etc. that
no longer exist).

Now also drops Folder rows whose path doesn't resolve under a mounted
source root, with the same defensive "skip if source root unmounted"
guard. The Settings orphan card surfaces both counts.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/services/api.ts

@@ -233,6 +233,8 @@ export interface RegenerateResult {
 export interface MissingStats {
   would_delete?: number
   deleted?: number
+  would_delete_folders?: number
+  deleted_folders?: number
   skipped_unmounted: number
   dry_run: boolean
 }