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/api/server.go

@@ -331,11 +331,11 @@ func (s *Server) handleValidatePair(w http.ResponseWriter, r *http.Request) {
 	in.apply(&p)
 	warns, err := s.eng.NormalizePair(&p)
 	if err != nil {
-		writeJSON(w, 200, map[string]any{"ok": false, "error": err.Error(), "warnings": []string{}})
+		writeJSON(w, 200, map[string]any{"ok": false, "error": err.Error(), "warnings": []store.Warning{}})
 		return
 	}
 	if warns == nil {
-		warns = []string{}
+		warns = []store.Warning{}
 	}
 	writeJSON(w, 200, map[string]any{"ok": true, "warnings": warns, "pair": p})
 }
@@ -374,7 +374,7 @@ func (s *Server) handleSavePair(w http.ResponseWriter, r *http.Request) {
 	}
 	p, _ = s.st.Pair(r.Context(), p.ID)
 	if warns == nil {
-		warns = []string{}
+		warns = []store.Warning{}
 	}
 	writeJSON(w, 200, map[string]any{"pair": s.view(r, p), "warnings": warns})
 }