"Modified" indicator "*" is unexpectedly still in title bar even when the project is no longer modified

OS and Version (please complete the following information):

• Operating System: Debian Trixie
• Shotcut Version: shotcut-linux-x86_64-26.2.26.txz from the project download page

Describe the bug
The modified indicator is unexpectedly still in title bar even when every change is undone.

Steps To Reproduce

1. Start Shotcut and open any project.
2. Add a clip, or perform any other action that leads to a history entry.
3. Undo the action with Ctrl-z, or undo the action manually in any other way (for instance by removing the clip again).
4. The modification indicator * in the title bar is still shown, indicating that the project is modified even though all modifications are undone. Hence, we get the impression that changes are made to the project, even though the current state is the same as the last saved state.

Expected behavior

The * indicator should not be present when the current project state is identical to the last saved state, so that it can be reliably used to see that a previously modified project has returned to the last saved state.

For instance, the current state of the project could be compared with the last saved state. Hashing could be used to keep the space overhead tiny, this is how for example GNU Emacs implements this feature.

Start by reproducing the issue in Shotcut on Debian Trixie: create a history entry, undo it, and check the title-bar indicator. Trace the project modified-state and title-bar update handling; done means the indicator clears when the current state matches the last saved state while remaining for unsaved changes.