fix: prevent OOM crash in audio waveform generation (#1501)

[AdityaDagar000]

Fixes #1501

What changed

• Added a 500 MB file size guard — skips waveform generation for very large files
• Peaks are extracted immediately after decode and the full AudioBuffer is released for GC
• AudioContext is properly closed after use to free Web Audio resources
• Added a cancelled flag to handle cleanup when the file changes mid-decode
• Added graceful error state

Root cause

file.arrayBuffer() was loading the entire file into memory, and the decoded
AudioBuffer was being held in state — causing OOM crashes on large audio files.

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