fix: Rename timer to stopwatch

Closes #392

client/src/models/Card.js

@@ -14,7 +14,7 @@ export default class extends BaseModel {
     name: attr(),
     description: attr(),
     dueDate: attr(),
-    timer: attr(),
+    stopwatch: attr(),
     isSubscribed: attr({
       getDefault: () => false,
     }),