fix: prevent infinite export loops for background music on audio-less videos

[Abhishek2005-ard]

Description

This PR resolves a critical issue where the FFmpeg export process could hang indefinitely or freeze the video frame when exporting a video that lacks an original audio track but has a background music track applied.

When hasOriginalAudio was false, the audio stream wasn't bounded by the amix filter's duration=first parameter, and the -shortest flag was missing. This resulted in Out-of-Memory (OOM) browser crashes when "Loop music" was enabled, and produced exports with frozen video tails when the music track was longer than the video clip.

Changes Made

• Added the apad filter to the background music input stream when hasOriginalAudio is false. This guarantees the audio stream is always at least as long as the video stream.
• Injected the -shortest flag into the FFmpeg execution arguments for video exports. This forces the encoder to safely terminate exactly when the video stream ends, gracefully preventing infinite loops and frozen frames.

Impact

This guarantees stability when adding custom music to silent videos. It completely prevents browser tab crashes caused by unbounded WASM memory usage and ensures output durations are perfectly matched.

Testing

• Verified logic with "Loop music" enabled on audio-less videos.
• Verified logic with music tracks longer than the original video duration.
• Verified logic with music tracks shorter than the original video duration.
• Passed all 71 existing test suites via npm test.

close #1577

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