Actually suspend the download process group on pause

Pause only stopped the stdout reader loop; yt-dlp kept transferring at
full rate until the pipe buffer filled, consuming bandwidth while the
UI claimed the download was paused. Send SIGSTOP/SIGCONT to the whole
process group (yt-dlp and its ffmpeg children) on Unix. Windows has no
equivalent signal; the limitation is documented in the helper.

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

ytsage/core/ytsage_downloader.py

@@ -839,9 +839,24 @@ class DownloadThread(QThread):
 
     def pause(self) -> None:
         self.paused = True
+        self._signal_process_group(signal.SIGSTOP if sys.platform != "win32" else None)
 
     def resume(self) -> None:
         self.paused = False
+        self._signal_process_group(signal.SIGCONT if sys.platform != "win32" else None)
+
+    def _signal_process_group(self, sig: Optional[int]) -> None:
+        """Send a signal to yt-dlp's whole process group (yt-dlp + ffmpeg children).
+
+        On Windows there is no SIGSTOP/SIGCONT; pausing there only stops output
+        consumption, which is a known limitation.
+        """
+        if sig is None or not self.process:
+            return
+        try:
+            os.killpg(os.getpgid(self.process.pid), sig)
+        except (ProcessLookupError, PermissionError, OSError) as e:
+            logger.debug(f"Could not signal process group: {e}")
 
     def cancel(self) -> None:
         self.cancelled = True