Diagram title input has no Escape-to-revert

[MrCoder]

Surfaced by the e2e gap suite (HDR-2, test.fixme in e2e/tests/header.spec.js).

Gap: the header title is a controlled <input> bound to setTitle on every keystroke (AppHeader.tsx → editorStore.setTitle); there is no draft buffer and no Escape/onKeyDown handler, so pressing Escape after editing does not revert. Common editors revert on Escape.

Fix: add a draft buffer + Escape-revert (and Enter-commit) to the title input. Then flip HDR-2 test.fixme → test.

[MrCoder]

Fixed in PR #828, shipped to prod in release-20260618082047. Title field now uses a draft buffer: Enter/blur commit, Escape reverts; HDR-2 e2e test flipped from fixme to real.