Groundwork before pulling logic out of App.svelte.
Vitest runs in a node environment. Nothing under test needs a DOM, because
every module takes its dependencies as arguments.
package-lock.json is tracked now and the Dockerfile installs with npm ci,
so the nightly rebuild can't quietly land on different versions.
.gitignore: .DS_Store was anchored to the root and missed nested copies,
and dist/ wasn't ignored at all.