Fix medium-severity defects across download, formats and tooling

- Format table: a missing acodec was treated as "has audio", skipping
  the +bestaudio merge and producing silent videos for extractors that
  omit the field.
- Progress bar: separate video/audio stream downloads each reported
  0-100%, making the bar jump backwards; per-phase scaling now maps the
  two streams onto 0-50/50-100.
- Custom commands: parse with shlex (quoted arguments with spaces were
  shredded by str.split), keep POSIX mode off on Windows so backslash
  paths survive, hide the console window like every other call site,
  close the stdout pipe, and support cancellation of a running command.
- Settings dialog: _("settings", "error_saving", ...) passed two
  positional args to the i18n helper, raising TypeError inside the
  except handler instead of showing the intended error dialog.
- ffmpeg on Windows: Path(os.getenv("LOCALAPPDATA")) crashed with
  TypeError when the variable is unset; fall back to the standard
  AppData/Local location.
- Version cache: cached path (str) was compared against a Path, so the
  cache never hit and every version query spawned a subprocess.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

ytsage/core/ytsage_downloader.py

@@ -113,6 +113,8 @@ class DownloadThread(QThread):
         self.subtitle_files: List[str] = []  # Track subtitle files that are created
         self.initial_subtitle_files: Set[Path] = set()  # Track initial subtitle files before download
         self.download_files: Set[Path] = set()  # Every destination path this download wrote to
+        self.expected_phases: int = 1  # 2 when video and audio download separately before merge
+        self._media_phase: int = 0  # Index of the media stream currently downloading
 
     def cleanup_partial_files(self) -> None:
         """Delete partial files (.part/.ytdl and unmerged .fNNN. streams), but only
@@ -293,6 +295,7 @@ class DownloadThread(QThread):
                 logger.debug(f"Using progressive format with bundled audio: {clean_format_id}")
             else:
                 cmd.extend(["-f", f"{clean_format_id}+bestaudio/best"])
+                self.expected_phases = 2  # separate video and audio downloads
                 logger.debug(f"Using video-only format merged with best audio: {clean_format_id}+bestaudio/best")
         else:
             # If no specific format ID, use resolution-based sorting (-S)
@@ -635,6 +638,8 @@ class DownloadThread(QThread):
                 filepath = dest_match.group(1).strip()
                 self.current_filename = Path(filepath).name
                 self.last_file_path = filepath  # Store the full path for later cleanup
+                if Path(filepath) not in self.download_files and Path(filepath).suffix.lower() not in SUBTITLE_EXTENSIONS:
+                    self._media_phase += 1
                 self.download_files.add(Path(filepath))
                 logger.debug(f"Extracted filename: {self.current_filename}")  # DEBUG
 
@@ -755,6 +760,11 @@ class DownloadThread(QThread):
         if percent_match:
             try:
                 percent = float(percent_match.group(1))
+                # When video and audio download as separate streams, scale each
+                # phase into its share of the bar instead of jumping 0-100 twice
+                if self.expected_phases > 1 and not self.is_playlist:
+                    completed = max(0, min(self._media_phase - 1, self.expected_phases - 1))
+                    percent = (completed * 100.0 + percent) / self.expected_phases
                 self.progress_signal.emit(percent)
             except (ValueError, IndexError):
                 pass