The code-review autosave skipped saving when the annotation list was empty
(`if (annotations.length === 0 && viewedFiles.size === 0) return`). So
deleting the last annotation never overwrote the stale draft on the server;
on refresh the draft-recovery dialog offered the deleted annotations back.
Mirror the plan-side useAnnotationDraft pattern: when the list is cleared,
DELETE the draft with a generation tombstone (so it can't resurface and a
late save can't revive it) instead of skipping. Guarded so an empty state
only triggers a delete once the user has actually authored annotations this
session — keyed on user-authored (non-source) annotations only, because
viewedFiles are seeded from GitHub on mount and external/SSE annotations have
their own lifecycle; counting either would let a fresh load or external churn
wrongly delete an unrestored draft. Also clear the pending autosave timer in
restoreDraft/dismissDraft so a stale save can't fire after restore/dismiss.
Adds real-stack regression tests (hook + disk layer via an /api/draft shim):
delete-to-empty removes the draft, a fresh/unengaged session preserves an
unrestored draft, and external-annotation churn issues no delete.
Reported by @alexanderkreidich.
Closes#948
Michael Ramos