Add unit tests for flattenMenuItems #20185

What

ui/src/utils/menuCustomization.ts already has a spec, but the exports below have no coverage in it.

The sidebar customisation dialog works off the flattened list this produces, so a nested item that gets lost here becomes an item the user cannot reorder or hide.

What to do

Add cases to the existing ui/tests/unit/utils/menuCustomization.spec.ts covering:

• a flat menu is returned unchanged
• children are flattened in with their parents, in display order
• arbitrarily deep nesting is fully flattened
• an item with an empty children array is kept, not dropped
• an empty menu returns an empty list

Reference

The spec file itself shows the house style; ui/tests/unit/utils/splitTranslation.spec.ts is another example.

How to run

Install once, then run the whole suite:

cd ui && npm install && npm run test:unit

While writing the test, run just this file so the feedback loop stays fast:

cd ui && npm run test:unit -- tests/unit/utils/menuCustomization.spec.ts

Proof required in the PR