chore: remove legacy python backend stub

Drop stale backend-era comment from wav export. The Python backend and
backend/ directory were already removed; no further stub files remain.

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/src/lib/audio/wav.ts

@@ -1,4 +1,4 @@
-/** Target sample rate for exported WAV files (matches former backend output). */
+/** Target sample rate for exported WAV files. */
 export const EXPORT_SAMPLE_RATE = 48000;
 
 /**