feat: disable Save button when content unchanged

- Save button starts disabled and only enables when title or description
  differs from the original task values
- updateSaveButton() compares current input values against task data
- input event listeners on title and desc fields call updateSaveButton
- Add global button:disabled style (opacity 0.5, cursor not-allowed)

static/css/style.css

@@ -182,6 +182,11 @@ body {
     background-color: #229954;
 }
 
+button:disabled {
+    opacity: 0.5;
+    cursor: not-allowed;
+}
+
 .error-message {
     color: #e74c3c;
     margin-bottom: 1rem;