fix: typo _find_ffmpeg_dir in update_cover_only (bump v1.9.5)

Il typo (underscore prefix) causava NameError nel thread upgrade
quando un file era già HQ (soglia superata) e serviva solo aggiornare
la copertina. Il thread moriva silenziosamente e l'utente vedeva
l'app 'bloccata' senza feedback.

core/config.py

@@ -5,7 +5,7 @@ import os
 import sys
 from pathlib import Path
 
-VERSION = "v1.9.4"
+VERSION = "v1.9.5"
 
 
 APP_NAME = "MusicTools"

core/upgrader.py

@@ -272,7 +272,7 @@ def update_cover_only(
         "--output", str(temp_dir / "cover"),
         video_url,
     ]
-    ffmpeg_dir = _find_ffmpeg_dir()
+    ffmpeg_dir = find_ffmpeg_dir()
     if ffmpeg_dir:
         cmd.extend(["--ffmpeg-location", ffmpeg_dir])
     if cookies_path and Path(cookies_path).exists():