Fix malformed Tailwind dark mode class in StopItem

[adityakulraj]

Summary

• replace invalid Tailwind class dark:bg[#000000] in src/components/StopItem.svelte
• use valid dark mode class dark:bg-black so dark background styling is applied

Validation

• git diff --check
• attempted npm run lint
  • blocked in this environment because npm install fails with internal npm error (Exit handler never called) and no local prettier binary is available

Closes #400

[CLAassistant]

All committers have signed the CLA.

[aaronbrethorst]

@adityakulraj you need to run npm run format on your machine, amend your existing commit, and force push the result up.