[FEATURE] Import/Export project file (.json) for programmatic workflows

Problem

OpenCut has no portable project file format. Projects are locked in IndexedDB/OPFS — users can't save, share, or move projects between devices. More importantly, external tools (AI assistants, CLI scripts, automation pipelines) have no way to programmatically generate an editing project and hand it to OpenCut for rendering.

Solution

A documented .json file format that captures the full editing state (tracks, clips, trim points, transitions, overlays, audio, project settings). Media files referenced by filename, not embedded.

• Export: button to save current project as .json
• Import: load .json file + source media, reconstruct timeline for preview and MP4 export
• Schema docs: publish a versioned JSON Schema so external tools can generate valid project files programmatically

Alternative

Expose a JS API for project manipulation instead of a file format. This would work for browser extensions but not for offline/external tool workflows.

Anything else?

Example use case: an AI assistant generates a .json file describing cuts, transitions, and overlays → user imports it with source media into OpenCut → previews, tweaks, exports MP4.

No files or tests are named. Start by tracing the existing IndexedDB/OPFS project persistence and the project/media import entry points; define the project JSON and versioned schema around the editing state described. Done means projects can be exported, imported with source media, previewed, and rendered to MP4.