Add cancel (discard) timer feature

Allows users to discard a running timer without creating a time entry.
A trash icon in the timer widget reveals a confirmation step ('Discard / Keep')
to prevent accidental data loss. Backend exposes a new DELETE /api/timer
endpoint that simply deletes the ongoingTimer row.

frontend/src/api/timer.ts

@@ -30,4 +30,8 @@ export const timerApi = {
     });
     return data;
   },
+
+  cancel: async (): Promise<void> => {
+    await apiClient.delete('/timer');
+  },
 };