[Feature] Undo/Redo & Version History - Non-Destructive Editing with Full History

[vzeman]

Summary

Implement non-destructive editing with comprehensive undo/redo and a named version snapshot system. Given that AI generation is expensive and time-consuming, users need confidence they can always revert to a known-good state.

Requirements

• Undo/redo for all scene editing actions (Ctrl+Z / Ctrl+Shift+Z)
• Undo history panel showing action list
• Version snapshots: manually save named versions ("v1 - first draft", "v2 - after feedback")
• Auto-snapshot before generation (so you can always revert to pre-generation state)
• Diff view between versions: what changed in each scene
• Restore any named version
• Branch versions: create alternate version from any snapshot
• Compare two versions side by side
• Version notes/comments
• Delete old versions to save space

Acceptance Criteria

• Ctrl+Z / Ctrl+Shift+Z undo/redo all editing actions
• Undo history panel lists all reversible actions
• Auto-snapshot is created automatically before each generation starts
• Named snapshots can be created, browsed, and restored
• Diff view highlights which scenes changed between two versions

Tech Notes

• Use command pattern for undo/redo stack
• Snapshots stored as lightweight diffs against base project state
• Generation outputs (video files) are not duplicated in snapshots — only references change

Related

Part of the ai-video-editor Flutter application. Works with #61 (Project Management) and #65 (Video Generation).