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_test.go

@@ -25,6 +25,7 @@ func TestMessageMethodsPlain(t *testing.T) {
 		{"Warning", func(w *Writer) { w.Warning("oops") }, "⚠️  Warning: oops\n"},
 		{"Error", func(w *Writer) { w.Error("boom") }, "🛑 ERROR: boom\n"},
 		{"Progress", func(w *Writer) { w.Progress("p") }, "  》 p\n"},
+		{"Detail", func(w *Writer) { w.Detail("d") }, "  》 d\n"},
 		{"Banner", func(w *Writer) { w.Banner("hello") }, "hello\n"},
 	}
 
@@ -39,6 +40,23 @@ func TestMessageMethodsPlain(t *testing.T) {
 	}
 }
 
+func TestWarningErrorCounters(t *testing.T) {
+	w, _ := newTestWriter(false)
+	if w.WarningCount() != 0 || w.ErrorCount() != 0 {
+		t.Fatalf("expected fresh writer to have zero counts")
+	}
+	w.Info("normal")
+	w.Warning("first warn")
+	w.Warning("second warn")
+	w.Error("only error")
+	if got, want := w.WarningCount(), 2; got != want {
+		t.Errorf("WarningCount: got %d, want %d", got, want)
+	}
+	if got, want := w.ErrorCount(), 1; got != want {
+		t.Errorf("ErrorCount: got %d, want %d", got, want)
+	}
+}
+
 func TestColorOutputContainsANSI(t *testing.T) {
 	w, buf := newTestWriter(true)
 	w.Error("boom")