feat: implement Audio Extraction (MP3/WAV) and self-host FFmpeg WASM engine

[Abhishek2005-ard]

Description

This PR introduces the Extract Audio feature, allowing users to rip just the audio track from their uploaded videos into MP3 or high-quality WAV formats.

Additionally, it fundamentally resolves the Failed to load the FFmpeg engine. Check your internet connection. error by self-hosting the FFmpeg WebAssembly assets locally, completely removing reliance on external CDNs like unpkg.com or jsdelivr during runtime.

✨ Key Features & Changes:

1. Audio Extraction Formats: Added MP3 and WAV options to the Format Selector dropdown.
2. Context-Aware UI: When an audio format is selected, the application automatically hides video-specific options (Quality sliders, Denoise, Stabilization) and displays a clean, informational banner.
3. Accurate Size Estimations: Updated the exportEstimate logic to accurately calculate the estimated file size for MP3 (192kbps) and WAV (CD-Quality 1411kbps) exports based on clip duration.
4. Self-Hosted FFmpeg Assets: Added scripts to pre-download the FFmpeg WebAssembly (.wasm), Web Worker, and core .js files into the public/ffmpeg directory.
5. Intelligent Hybrid Loader: Re-engineered src/lib/ffmpeg.ts to seamlessly pick the best local FFmpeg build based on browser isolation:
   • Uses the extremely fast Multi-Threaded (core-mt) build when the environment is fully Cross-Origin Isolated.
   • Falls back to the universally compatible Single-Threaded UMD build when running in non-isolated local development environments, completely preventing Chromium strict-sandbox Web Worker crashes.
6. Local Development Isolation: Added COOP (same-origin) and COEP (require-corp) HTTP headers to next.config.ts to mimic production isolation locally.

🧪 How to Test:

1. Start the local development server (npm run dev).
2. Upload a video file.
3. Scroll down and select MP3 or WAV in the Output Format dropdown.
4. Verify that the video quality sliders are hidden and replaced with an audio-only notice.
5. Click EXPORT and ensure the process finishes correctly without any "FFmpegLoadError" or network connection issues, and that the audio file downloads successfully.

ScrrenRecord:-

Screen.Recording.2026-05-24.212116.mp4

close #1079

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👋 Thanks for your PR, @Abhishek2005-ard!

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

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