Add audio format selection for audio-only downloads

Introduces settings and UI for forcing audio format conversion on audio-only downloads, including support for selecting preferred audio formats. Updates language files for all supported languages, adds config options, and passes new settings through the downloader and main app logic.

src/core/ytsage_downloader.py

@@ -61,6 +61,8 @@ class DownloadThread(QThread):
         geo_proxy_url=None,
         force_output_format=False,
         preferred_output_format="mp4",
+        force_audio_format=False,
+        preferred_audio_format="best",
     ) -> None:
         super().__init__()
         self.url = url
@@ -86,6 +88,8 @@ class DownloadThread(QThread):
         self.geo_proxy_url = geo_proxy_url
         self.force_output_format = force_output_format
         self.preferred_output_format = preferred_output_format
+        self.force_audio_format = force_audio_format
+        self.preferred_audio_format = preferred_audio_format
         self.paused: bool = False
         self.cancelled: bool = False
         self.process: Optional[subprocess.Popen] = None
@@ -184,7 +188,7 @@ class DownloadThread(QThread):
             res_value: str = self.resolution if self.resolution else "720"  # Default to 720p if no resolution specified
             cmd.extend(["-S", f"res:{res_value}"])
 
-        # Force output format if enabled and merging is needed
+        # Force output format if enabled and merging is needed (for video)
         if self.force_output_format and not self.is_audio_only:
             if self.format_has_audio:
                 # Progressive format (video with audio) - use remux to convert container
@@ -195,6 +199,15 @@ class DownloadThread(QThread):
                 cmd.extend(["--merge-output-format", self.preferred_output_format])
                 logger.debug(f"Using --merge-output-format to force merged format to: {self.preferred_output_format}")
 
+        # Force audio format conversion for audio-only downloads
+        if self.is_audio_only and self.force_audio_format:
+            cmd.append("--extract-audio")
+            if self.preferred_audio_format and self.preferred_audio_format != "best":
+                cmd.extend(["--audio-format", self.preferred_audio_format])
+                logger.debug(f"Using --extract-audio with --audio-format {self.preferred_audio_format} for audio-only download")
+            else:
+                logger.debug("Using --extract-audio with best quality (no conversion) for audio-only download")
+
         # Output template with resolution in filename
         # Use string concatenation instead of Path.joinpath to avoid Path object issues
         base_path: str = self.path.as_posix()