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/gui/ytsage_gui_format_table.py

@@ -346,7 +346,10 @@ class FormatTableMixin:
             checkbox.setStyleSheet("QCheckBox { margin-left: 8px; }")
             checkbox.format_id = f["format_id"]
             checkbox.is_audio_only = f.get("vcodec") == "none"
-            checkbox.has_audio = f.get("acodec") != "none"
+            # A missing acodec means unknown, not progressive - treating it as
+            # has-audio skips the +bestaudio merge and yields silent videos
+            acodec = f.get("acodec")
+            checkbox.has_audio = acodec is not None and acodec != "none"
             checkbox.clicked.connect(lambda checked, cb=checkbox: self.handle_checkbox_click(cb))
             self.format_checkboxes.append(checkbox)