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/store/models.go

@@ -91,25 +91,61 @@ func (s *Store) PruneSessions(ctx context.Context) {
 	_, _ = s.DB.ExecContext(ctx, `DELETE FROM sessions WHERE expires_at <= ?`, now())
 }
 
+// ---- warnings ----
+
+// Warning is a scan or validation notice, structured so the UI can lay it
+// out (a heading, an explanation and a table of affected folders) rather
+// than print one long sentence.
+type Warning struct {
+	Title  string        `json:"title"`
+	Detail string        `json:"detail,omitempty"`
+	Items  []WarningItem `json:"items,omitempty"`
+	// Total is the full number of items when Items was truncated.
+	Total int `json:"total,omitempty"`
+}
+
+// WarningItem pairs a slave path with the master path it corresponds to.
+type WarningItem struct {
+	Slave  string `json:"slave"`
+	Master string `json:"master"`
+	Count  int    `json:"count,omitempty"` // songs affected, when grouped
+}
+
+// decodeWarnings reads stored warnings; scans before structured warnings
+// stored plain strings, which become title-only warnings.
+func decodeWarnings(raw string) []Warning {
+	var ws []Warning
+	if json.Unmarshal([]byte(raw), &ws) == nil && ws != nil {
+		return ws
+	}
+	var old []string
+	_ = json.Unmarshal([]byte(raw), &old)
+	ws = make([]Warning, 0, len(old))
+	for _, t := range old {
+		ws = append(ws, Warning{Title: t})
+	}
+	return ws
+}
+
 // ---- pairs ----
 
 type Pair struct {
-	ID                  int64    `json:"id"`
-	Name                string   `json:"name"`
-	MasterRoot          string   `json:"master_root"`
-	SlaveRoot           string   `json:"slave_root"`
-	QuarantineDir       string   `json:"quarantine_dir"`
-	Extensions          []string `json:"extensions"`
-	AutoLinkIdentical   bool     `json:"auto_link_identical"`
-	ScanIntervalMinutes int      `json:"scan_interval_minutes"`
-	RetentionDays       int      `json:"retention_days"`
-	SettleMinutes       int      `json:"settle_minutes"`
-	ApproxMatching      bool     `json:"approx_matching"`
-	ApproxThreshold     int      `json:"approx_threshold"` // percent
-	LastScanAt          *int64   `json:"last_scan_at"`
-	LastScanError       string   `json:"last_scan_error"`
-	LastScanWarnings    []string `json:"last_scan_warnings"`
-	CreatedAt           int64    `json:"created_at"`
+	ID                  int64     `json:"id"`
+	Name                string    `json:"name"`
+	MasterRoot          string    `json:"master_root"`
+	SlaveRoot           string    `json:"slave_root"`
+	QuarantineDir       string    `json:"quarantine_dir"`
+	Extensions          []string  `json:"extensions"`
+	AutoLinkIdentical   bool      `json:"auto_link_identical"`
+	ScanIntervalMinutes int       `json:"scan_interval_minutes"`
+	RetentionDays       int       `json:"retention_days"`
+	SettleMinutes       int       `json:"settle_minutes"`
+	ApproxMatching      bool      `json:"approx_matching"`
+	ApproxThreshold     int       `json:"approx_threshold"` // percent
+	LastScanAt          *int64    `json:"last_scan_at"`
+	LastScanError       string    `json:"last_scan_error"`
+	LastScanWarnings    []Warning `json:"last_scan_warnings"`
+	CreatedAt           int64     `json:"created_at"`
 }
 
 const pairCols = `id, name, master_root, slave_root, quarantine_dir, extensions, auto_link_identical,
@@ -132,10 +168,7 @@ func scanPair(r scanner) (Pair, error) {
 		p.LastScanAt = &last.Int64
 	}
 	p.Extensions = strings.Split(exts, ",")
-	_ = json.Unmarshal([]byte(warns), &p.LastScanWarnings)
-	if p.LastScanWarnings == nil {
-		p.LastScanWarnings = []string{}
-	}
+	p.LastScanWarnings = decodeWarnings(warns)
 	return p, nil
 }
 
@@ -196,9 +229,9 @@ func (s *Store) ClearMatches(ctx context.Context, pairID int64) error {
 	return err
 }
 
-func (s *Store) FinishScan(ctx context.Context, pairID, at int64, scanErr string, warnings []string) error {
+func (s *Store) FinishScan(ctx context.Context, pairID, at int64, scanErr string, warnings []Warning) error {
 	if warnings == nil {
-		warnings = []string{}
+		warnings = []Warning{}
 	}
 	w, _ := json.Marshal(warnings)
 	_, err := s.DB.ExecContext(ctx, `UPDATE pairs SET last_scan_at = ?, last_scan_error = ?, last_scan_warnings = ? WHERE id = ?`,