Add unit tests for the log display preferences #20188

What

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

These four preferences persist how the logs view renders. Five of the module's ten exports are uncovered, including every one of the persisted refs.

What to do

Add cases to the existing ui/tests/unit/composables/useLogDisplay.spec.ts covering:

• each preference reads its stored value on first access
• each preference writes back when changed
• a missing stored value falls back to the documented default
• a malformed stored value falls back to the default instead of throwing
• the same preference shared between two callers stays in sync
• DENSITY_PADDING has an entry for every density the picker offers

Reference

The spec file itself shows the house style; ui/tests/unit/composables/useTenant.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/composables/useLogDisplay.spec.ts

Proof required in the PR