fix(audio): prevent tail truncation during video export

[iMyth]

Problem

Audio gets cut off at the end when exporting video, both with and without sleep mode enabled.

Root Cause

1. loudnorm filter missing linear=true — default mode uses an internal lookahead buffer that drops trailing samples at EOF
2. -shortest flag truncates to the shorter stream — when video is slightly shorter than audio (due to frame duration rounding), the audio tail gets clipped

Fix

• Add linear=true to loudnorm in both sleep mode and normal mode, ensuring fixed gain is applied without altering duration
• Replace -shortest in merge_video_with_audio with -t <audio_duration> (probed via ffprobe), making audio the authoritative length source
• Add probe_duration_secs helper function

Scope

• Single section export
• Multi-section merge
• Sleep mode audio processing