Refactor to enforce strictNullChecks

This commit applies `strictNullChecks` to the entire codebase to improve
maintainability and type safety. Key changes include:

- Remove some explicit null-checks where unnecessary.
- Add necessary null-checks.
- Refactor static factory functions for a more functional approach.
- Improve some test names and contexts for better debugging.
- Add unit tests for any additional logic introduced.
- Refactor `createPositionFromRegexFullMatch` to its own function as the
  logic is reused.
- Prefer `find` prefix on functions that may return `undefined` and
  `get` prefix for those that always return a value.

src/presentation/components/Scripts/View/Cards/CardSelectionIndicator.vue

@@ -34,7 +34,7 @@ export default defineComponent({
     const currentCollection = computed<ICategoryCollection>(() => currentState.value.collection);
 
     const currentCategory = computed<ICategory>(
-      () => currentCollection.value.findCategory(props.categoryId),
+      () => currentCollection.value.getCategory(props.categoryId),
     );
 
     const isAnyChildSelected = computed<boolean>(