Show scan warnings as structured cards instead of long sentences

Warnings are now stored and served as structured data (title, detail,
slave/master items, total) and rendered as cards with a Slave -> Master
table: the first five rows, with "Show all" for the rest.

- Renamed folders highlight the characters that differ; non-ASCII
  characters get their code point in a tooltip, so look-alikes such as
  "-" (U+002D) and "‐" (U+2010) can be told apart.
- Songs in a different folder are grouped by folder pair with a song
  count, most affected first, instead of two long examples.
- Folders that only differ by name are no longer also reported as
  songs in a different folder.
- Warnings saved as plain text by earlier scans still display.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/engine/engine_test.go

@@ -502,8 +502,8 @@ func TestAnyLayoutMatchesWithoutWarnings(t *testing.T) {
 		t.Fatal(err)
 	}
 	for _, w := range p.LastScanWarnings {
-		if strings.Contains(w, "layout") || strings.Contains(w, "folder") || strings.Contains(w, "depth") {
-			t.Fatalf("matching layouts must not produce layout warnings, got %q", w)
+		if text := w.Title + " " + w.Detail; strings.Contains(text, "layout") || strings.Contains(text, "folder") || strings.Contains(text, "depth") {
+			t.Fatalf("matching layouts must not produce layout warnings, got %+v", w)
 		}
 	}
 }