fix: display only real sync errors in error count and list

- Added CountErroredChanges() method to count only pending changes with LastError
- Previously, error count included all pending changes, not just failed ones
- Added /api/sync/info endpoint with proper error count and error list
- Added sync info modal to display sync status, error count, and error details
- Made sync status indicators clickable to open the modal
- Fixed disconnect between "Error count: 4" and "No errors" in the list

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

internal/localdb/localdb.go

@@ -396,6 +396,13 @@ func (l *LocalDB) CountPendingChangesByType(entityType string) int64 {
 	return count
 }
 
+// CountErroredChanges returns the number of pending changes with errors
+func (l *LocalDB) CountErroredChanges() int64 {
+	var count int64
+	l.db.Model(&PendingChange{}).Where("last_error != ?", "").Count(&count)
+	return count
+}
+
 // MarkChangesSynced marks multiple pending changes as synced by deleting them
 func (l *LocalDB) MarkChangesSynced(ids []int64) error {
 	if len(ids) == 0 {