feat: add audio fade controls

[saurabhhhcodes]

Closes #123\n\nAdd fade-in and fade-out controls to the audio panel, persist them in the recipe, and apply the fades in the FFmpeg audio chain with trim-aware timing.\n\nValidation:\n- bunx vitest run src/lib/tests/ffmpeg.test.ts src/lib/tests/editorPersistence.test.ts\n- bunx tsc -p tsconfig.json --noEmit\n- git diff --check

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