Fix #112: Add support for opting into beta channel of releases

Issue: Allow users to test completed builds without creating entire releases.

Solution:
- Added beta_channel parameter to UpdateCheckThread.__init__()
- Modified get_latest_release_for_platform() to respect beta_channel flag:
  * Stable (default): Only stable releases (no prerelease, no draft)
  * Beta opt-in: Includes prerelease builds, only excludes drafts
- Supports FLARES_BETA_CHANNEL environment variable (1/true/yes to enable)

Usage examples:
  export FLARES_BETA_CHANNEL=1 && ./flares.sh
  set FLARES_BETA_CHANNEL=true && flares.exe

Benefits:
  ✓ Easy opt-in via environment variable (no UI changes needed)
  ✓ No breaking changes - stable remains default
  ✓ Draft builds always excluded (security measure)
  ✓ Prerelease detection uses Gitea API 'prerelease' flag

updater.py

@@ -159,6 +159,7 @@ class UpdateCheckThread(QThread):
         current_version: str,
         platform_name: str,
         app_name: str,
+        beta_channel: bool = False,  # Issue #112: Beta channel support
     ) -> None:
 
         super().__init__()
@@ -167,6 +168,7 @@ class UpdateCheckThread(QThread):
         self.current_version = current_version
         self.platform_name = platform_name
         self.app_name = app_name
+        self.beta_channel = beta_channel or os.environ.get("FLARES_BETA_CHANNEL", "").lower() in ("1", "true", "yes")
 
     def run(self) -> None:
         try:
@@ -198,7 +200,13 @@ class UpdateCheckThread(QThread):
                 if not releases:
                     continue
 
-                latest = next((r for r in releases if not r.get("prerelease") and not r.get("draft")), None)
+                # Issue #112: Support beta channel opt-in
+                if self.beta_channel:
+                    # Beta users: include prerelease builds, prefer newest non-draft
+                    latest = next((r for r in releases if not r.get("draft")), None)
+                else:
+                    # Stable users: only stable releases (no prerelease, no draft)
+                    latest = next((r for r in releases if not r.get("prerelease") and not r.get("draft")), None)
 
                 if not latest:
                     continue
@@ -331,7 +339,15 @@ class UpdateManager(QObject):
             self.start_update_check_thread()
 
     def start_update_check_thread(self) -> None:
-        self.check_thread = UpdateCheckThread(self.api_url, self.api_url_sec, self.current_version, self.platform_name, self.app_name)
+        # Issue #112: Check environment variable for beta channel opt-in
+        self.check_thread = UpdateCheckThread(
+            self.api_url, 
+            self.api_url_sec, 
+            self.current_version, 
+            self.platform_name, 
+            self.app_name,
+            beta_channel=os.environ.get("FLARES_BETA_CHANNEL", "").lower() in ("1", "true", "yes")
+        )
         self.check_thread.download_requested.connect(self.on_server_update_requested)
         self.check_thread.no_update_available.connect(self.on_server_no_update)
         self.check_thread.error_occurred.connect(self.on_error)