Editor fires an autosave immediately on open

[dubadub]

Found during review of #456. Pre-existing on main.

Opening /edit/<recipe> shows "Saved" in the header with no edits made, meaning the autosave PUT runs on load (templates/edit.html, the onChange wiring near the editor initialisation). This rewrites the file's mtime on every open, which also affects the index's modified-date sort.

Fix: only schedule autosave when the content differs from originalContent, and do not treat the initial editor population as a change.