Add opt-in beta update UI and checks

Introduce an opt-in beta updates feature: add a styled "Receive Beta Updates" checkbox in the Updater UI with i18n strings, load/save logic in the custom options dialog, and a default config key (check_beta_updates=false).

Add _fetch_github_beta_version to the update thread to query GitHub releases (selecting the highest tag) and emit update_available for newer beta builds. When beta checking is enabled, the thread will perform the GitHub beta check and return early to avoid conflicting with the PyPI check. This enables users to opt into preview releases safely.

ytsage/gui/ytsage_gui_main.py

@@ -116,9 +116,63 @@ class UpdateCheckThread(QThread):
             # Silently fallback if GitHub API fails (rate limiting, network issues, etc.)
             return fallback
 
+    def _fetch_github_beta_version(self) -> tuple[str | None, str | None, str | None]:
+        """Fetch latest version code from GitHub releases (including betas). Returns (version, tag, changelog)."""
+        try:
+            response = requests.get(
+                "https://api.github.com/repos/oop7/YTSage/releases",
+                headers={"Accept": "application/vnd.github.v3+json"},
+                timeout=self.GITHUB_TIMEOUT,
+            )
+            if response.status_code != 200:
+                logger.debug(f"GitHub Releases API returned {response.status_code}")
+                return None, None, None
+
+            releases = response.json()
+            if not releases:
+                return None, None, None
+
+            latest_release = None
+            highest_ver = version.parse("0.0.0")
+
+            for rel in releases:
+                tag = rel.get("tag_name", "")
+                ver_str = tag.lstrip("v")
+                try:
+                    v = version.parse(ver_str)
+                    if v > highest_ver:
+                        highest_ver = v
+                        latest_release = rel
+                except Exception:
+                    continue
+            
+            if latest_release:
+                return str(highest_ver), latest_release.get("tag_name"), latest_release.get("body")
+            return None, None, None
+
+        except Exception as e:
+            logger.debug(f"GitHub beta check error: {e}")
+            return None, None, None
+
     def run(self):
         """Check for updates using parallel network requests for better performance."""
         try:
+            # Check for beta updates if enabled
+            check_beta = ConfigManager.get("check_beta_updates")
+            
+            if check_beta:
+                latest_ver_str, tag, changelog = self._fetch_github_beta_version()
+                
+                if latest_ver_str and version.parse(latest_ver_str) > version.parse(self.current_version):
+                    release_url = f"https://github.com/oop7/YTSage/releases/tag/{tag}"
+                    if not changelog:
+                        changelog = "View the full changelog on GitHub."
+                    self.update_available.emit(latest_ver_str, release_url, changelog)
+                # Return if beta check completes (whether update found or not), 
+                # effectively skipping PyPI check if beta is enabled. 
+                # This ensures we don't downgrade or conflict.
+                return
+
             # Use ThreadPoolExecutor to make both requests in parallel
             # This reduces total wait time from potentially 15s to ~8s max
             with ThreadPoolExecutor(max_workers=2) as executor: