Indent snapshot summary details; add Finished message; fix 'to process'

- New ui.Detail method for indented continuation lines under a
  preceding Complete (visually same as Progress: "  》" in white).
- Snapshot summary lines (Files/Data/Storage/Upload/Duration) are
  now Detail lines indented under "Created snapshot X.".
- Local index database prune complete result lines (incomplete
  snapshots, orphaned files/chunks/blobs) are also Detail lines
  under a clean Complete header.
- "Files: ... to process" → "Files: ... processed" (they have been
  processed by the time we emit the summary).
- "Data: ... (... to process)" → "Data: ... (... processed)".
- ui.Writer now tracks warning and error counts emitted; Vaultik
  prints "Finished successfully." or "Finished (with N warnings)."
  as the final line of CreateSnapshot.

README.md

@@ -407,6 +407,7 @@ Message classes:
 | Warning | `⚠️  Warning:` (orange/yellow) | column 0 | Recoverable problem |
 | Error | `🛑 ERROR:` (red) | column 0 | Operation aborted |
 | Progress | `  》` (white) | column 2 | Heartbeat or per-item status during a long-running operation |
+| Detail | `  》` (white) | column 2 | Continuation/sub-line of a preceding Complete (visually identical to Progress) |
 
 Conventions: