Add Deno integration check and system info thread

Introduce check_ytdlp_deno_integration() in ytsage_yt_dlp.py to detect Deno integration by running `yt-dlp --verbose` and scanning debug output for JS runtimes containing "deno" (with timeout and logging/fallback).

Add SystemInfoThread (QThread) in ytsage_dialogs_base.py to collect yt-dlp, ffmpeg and deno presence, versions, paths, cache timestamps and integration status in the background, emitting the gathered info via info_ready. AboutDialog now starts the thread in update_system_info() and populates the UI asynchronously via _populate_system_info(), showing a small "+ yt-dlp" integration indicator next to the Deno status when detected. Refactors usage of version/cache lookups to use the thread-provided info dictionary.

ytsage/core/ytsage_yt_dlp.py

@@ -712,3 +712,43 @@ def setup_ytdlp(parent_widget=None):
     # User cancelled or setup failed, return the fallback command
     logger.debug("Returning fallback command 'yt-dlp'")
     return "yt-dlp"
+
+
+def check_ytdlp_deno_integration() -> bool:
+    """
+    Check if yt-dlp is integrated with Deno by running 'yt-dlp --verbose'.
+    Returns:
+        bool: True if Deno is detected in JS runtimes, False otherwise
+    """
+    try:
+        ytdlp_path = get_yt_dlp_path()
+        if not ytdlp_path or ytdlp_path == "yt-dlp":
+            return False
+
+        # Run yt-dlp --verbose to check JS runtimes
+        # We use a dummy URL or just --verbose with no URL (which might error but should print debug info)
+        # However, yt-dlp might not print debug info if no URL is provided and it errors out immediately with "usage".
+        # But per user example: "yt-dlp.exe: error: You must provide at least one URL." comes AFTER debug info.
+        
+        result = subprocess.run(
+            [str(ytdlp_path), "--verbose"],
+            capture_output=True,
+            text=True,
+            timeout=10,
+            creationflags=SUBPROCESS_CREATIONFLAGS
+        )
+        
+        # Check stderr for "[debug] JS runtimes: deno"
+        output = result.stderr
+        if "[debug] JS runtimes:" in output and "deno" in output:
+             # Find the line
+            for line in output.splitlines():
+                if "[debug] JS runtimes:" in line and "deno" in line:
+                    logger.info(f"Deno integration detected: {line.strip()}")
+                    return True
+        
+        return False
+            
+    except Exception as e:
+        logger.warning(f"Failed to check yt-dlp Deno integration: {e}")
+        return False