remove dead EditTask and dedupe the CLI/TUI sort + render logic

Two bits of cleanup in the same area:

EditTask in actions.go is defined with a doc comment but never called from anywhere — the TUI edits tasks through $EDITOR instead. Delete it or wire it up.

The sort comparator and the due-date colour logic are copy-pasted between main.go's listTasks and tui.go's refreshTasks/View. They've already drifted — the CLI version has no terminal-width truncation — and they have to be kept in sync by hand. Hoist the shared sort/format helpers into one place.