fix: delete underlying FIT file when deleting a workout from history (#4788)

Deleting a workout only removed its row from the SQLite database, leaving
the .fit file on disk. FitDatabaseProcessor::processDirectory() rescans
that directory on every app start and re-imports any file whose hash isn't
already in the DB, so deleted workouts silently reappeared after restart.

Verified on-device: end 3 workouts, delete one, restart the app -> before
this fix it came back, after this fix it stays deleted.

Author: cagnulein

src/workoutmodel.cpp

@@ -138,10 +138,11 @@ bool WorkoutModel::deleteWorkout(int workoutId) {
         return false;
     }
 
-    // Optionally, you could also delete the FIT file here if desired
-    // if (!filePath.isEmpty()) {
-    //     QFile::remove(filePath);
-    // }
+    // Delete the underlying FIT file too, otherwise FitDatabaseProcessor::processDirectory()
+    // will find it unindexed on the next app start and re-insert it into the workouts table.
+    if (!filePath.isEmpty()) {
+        QFile::remove(filePath);
+    }
 
     // Refresh the model
     refresh();