fix: preserve background music when original video audio is muted

[Abhishek2005-ard]

Description

This PR resolves a frustrating UX bug where muting the original video audio (by toggling keepAudio to false) would accidentally completely discard any custom background music the user had uploaded.

Previously, the detection for a background music track strictly required recipe.keepAudio to be true, causing the entire audio pipeline to be shut down during export if the video was muted.

Changes Made

• Decoupled hasMusicTrack from recipe.keepAudio so the editor correctly detects background music regardless of the video's mute state.
• Updated the shouldKeepAudio aggregate check in buildArguments to evaluate to true if either the original audio is kept or a background music track is present: (recipe.keepAudio && hasOriginalAudio) || hasMusicTrack.
• Adjusted the internal hasOriginalAudio conditions inside the filter_complex builder to explicitly verify recipe.keepAudio. If keepAudio is false, it routes to the else block and processes the background music as the sole audio output stream.

Impact

Users can now confidently mute their noisy original video footage while retaining their uploaded background music track, resulting in a clean, perfectly mixed final export.

Testing

• Tested export with both original audio and background music enabled.
• Tested export with original video audio muted (keepAudio: false) and background music enabled (successfully exported with only background music).
• Passed all 71 existing test suites via npm test.

close #1759

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[magic-peach]

Thanks for opening this! This PR has picked up merge conflicts with main — the codebase has moved a fair amount recently (a Next.js version upgrade and a couple of infra fixes). Rather than leave it conflicting indefinitely, I'm closing it for now. If you'd like to pick "fix: preserve background music when original video audio is muted" back up, a fresh rebase (or a new PR off current main) would be very welcome — the idea itself isn't being rejected, just the current diff.