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.

internal/ui/ui.go

@@ -46,9 +46,14 @@ const (
 const Marker = "》"
 
 // Writer formats and emits user-facing messages with optional ANSI color.
+// It also counts warnings and errors emitted so the caller can summarize at
+// the end of an operation ("Finished successfully." vs "Finished with
+// warnings.").
 type Writer struct {
-	out   io.Writer
-	color bool
+	out      io.Writer
+	color    bool
+	warnings int
+	errors   int
 }
 
 // New returns a Writer that emits to out. Color is enabled when out is a
@@ -115,6 +120,7 @@ func (w *Writer) Notice(format string, args ...any) {
 
 // Warning prints "⚠️  Warning: " in orange/yellow followed by the message.
 func (w *Writer) Warning(format string, args ...any) {
+	w.warnings++
 	prefix := "⚠️  " + w.paint(ansiYellow+ansiBold, "Warning: ")
 	_, _ = fmt.Fprintln(w.out, prefix+fmt.Sprintf(format, args...))
 }
@@ -123,10 +129,25 @@ func (w *Writer) Warning(format string, args ...any) {
 // same writer as everything else; callers that want stderr should
 // construct a separate Writer for it.
 func (w *Writer) Error(format string, args ...any) {
+	w.errors++
 	prefix := "🛑 " + w.paint(ansiRed+ansiBold, "ERROR: ")
 	_, _ = fmt.Fprintln(w.out, prefix+fmt.Sprintf(format, args...))
 }
 
+// Detail prints an indented continuation line under a preceding Complete
+// (or other top-level message). Marker "  》" (white) at column 2.
+// Distinct from Progress (semantically a "heartbeat") in usage but
+// visually identical.
+func (w *Writer) Detail(format string, args ...any) {
+	w.emit(ansiWhite, "  "+Marker, "", format, args)
+}
+
+// WarningCount returns the number of Warning() calls this writer has emitted.
+func (w *Writer) WarningCount() int { return w.warnings }
+
+// ErrorCount returns the number of Error() calls this writer has emitted.
+func (w *Writer) ErrorCount() int { return w.errors }
+
 // Progress prints an indented heartbeat / per-item update, marker in white.
 func (w *Writer) Progress(format string, args ...any) {
 	w.emit(ansiWhite, "  "+Marker, "", format, args)