Replaces checkAndMigrateTasks, which never ran. Its guard compared
currentDate, set from new Date() at init, against a fresh new Date() a few
milliseconds later in onMount, so it was never true. Unfinished tasks have
been sitting under old date keys ever since.
The rule now: unfinished tasks from every prior day move into today,
oldest first, ahead of whatever is already there. Completed ones are
dropped and the old key deleted. It spans gaps, which the original never
did even in theory, since it only looked one day back.
now is a parameter. That's the part that makes the boundary testable.
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.