feat: highlight overdue todos with red background and icon

MindDump/Views/TodoRowView.swift

@@ -5,6 +5,13 @@ struct TodoRowView: View {
     let onToggle: () -> Void
     var onTap: (() -> Void)?
 
+    private var isOverdue: Bool {
+        !item.isCompleted && item.deadline != nil
+            && item.deadline! < Calendar.current.startOfDay(
+                for: Calendar.current.date(byAdding: .day, value: 1, to: Date())!
+            )
+    }
+
     var body: some View {
         HStack {
             Button(action: onToggle) {
@@ -37,8 +44,15 @@ struct TodoRowView: View {
                     .font(.system(size: 8))
                     .foregroundStyle(priority.color)
             }
+
+            if isOverdue {
+                Image(systemName: "exclamationmark.circle.fill")
+                    .font(.system(size: 14))
+                    .foregroundStyle(.red)
             }
         }
+        .listRowBackground(isOverdue ? Color.red.opacity(0.08) : nil)
+    }
 
     private func noteSnippet(_ text: String) -> String {
         if text.count > 50 {