Feature: Section save (heart toggle) and take file cleanup

[amitayks]

Summary

Two related capabilities for managing timeline sections and disk space:

Section Save (Heart Toggle)

• Each section in the sidebar gets a heart icon to mark it as "saved"
• Three section states: Active (in timeline), Saved+Removed (hearted but removed from timeline, grayed out with [+] re-add button), Deleted (gone entirely)
• Saved sections persist in timeline.savedSections in project.json
• Both active and saved sections appear in the sidebar sorted by time, with saved sections grayed out
• Full undo/redo support for delete and re-add operations

Take File Cleanup

• Deleting a section checks if its take is still referenced by any remaining active or saved section
• If no references remain, the take's recording files (.webm) are moved to a .deleted/ staging folder inside the project directory
• .deleted/ is permanently cleaned up on: project open, project switch, and via a "Save & Clean" button on the home screen
• File moves to/from .deleted/ are reversed on undo
• Prevents unbounded disk usage from orphaned recording files

Why

Previously, deleting a section removed it from the project data but left the take's recording files on disk. There was no way to preserve useful takes for future use while cleaning up unwanted ones.

Related Commits

• 7d0c576 — feat: add section save/heart toggle and take file cleanup

Fixes included in PR #3