feat: background rendering queue for exports

[mrdeyroy]

Description

This PR introduces a background rendering queue for video exports, resolving #1253. It unblocks the main editor UI during exports, allowing users to continue interacting with the app, change settings, or upload new files while FFmpeg processes videos in the background.

Closes

Closes #1253

Changes Included

• Global Export Context: Implemented ExportQueueContext which acts as the single source of truth for all export jobs. It manages state globally rather than being tied to the editor lifecycle.
• Floating Queue UI: Added ExportQueuePanel, a floating panel that tracks the progress of queued, active, completed, and failed jobs. Users can minimize/maximize this panel, cancel jobs, and download their finished videos directly from here.
• Sequential Mutex Processing: FFmpeg jobs are now processed sequentially via a queue mutex. This ensures the browser does not run out of memory or crash the CPU by attempting parallel ffmpeg.wasm operations.
• Decoupled Editor UI: Removed the blocking full-screen ExportOverlay and inline DownloadResult from VideoEditor.tsx.
• Hooks Refactor: Cleaned up useVideoEditor.ts to no longer handle individual local export tracking, shifting responsibilities to the central queue manager.

How to Test

1. Upload a video file into the editor.
2. Hit the "Export" button. Observe that the main editor UI remains unlocked.
3. Check the floating queue panel in the bottom right corner—it should accurately reflect the active export progress.
4. Upload a second video and initiate another export while the first is running. Ensure the second job is added to the "Queued" list and begins strictly after the first one finishes.
5. Once a job is complete, test the download button within the queue panel.
6. Trigger an export and hit the cancel ("X") button in the queue to verify termination of the FFmpeg process.

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Welcome to Reframe — a browser-based video editor built for everyone 🎬

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Quick checklist

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