The web app reopens the last visit's project and, when it had unsaved changes, marked it
modified with `bump()`. Since M3.9 the modified mark follows the saved snapshot, and a bump on
the very project just opened counts as "back at the saved state", so restored changes looked
saved (no `*`, no unsaved-changes prompt). `Session::mark_unsaved` forgets the saved snapshot
instead: the project stays modified until saved, whatever is undone. Regression test included.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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