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_dialogs/ytsage_dialogs_updater.py

@@ -441,6 +441,40 @@ class UpdaterTabWidget(QWidget):
         deno_layout.addLayout(deno_button_layout)
         
         layout.addWidget(deno_group)
+
+        # === App Updates Section ===
+        app_update_group = QGroupBox(_("settings.app_updates_title"))
+        app_update_layout = QVBoxLayout()
+        
+        self.beta_updates_checkbox = QCheckBox(_("settings.check_beta_updates"))
+        self.beta_updates_checkbox.setStyleSheet(
+            """
+            QCheckBox {
+                color: #ffffff;
+                spacing: 5px;
+                padding: 3px;
+            }
+            QCheckBox::indicator {
+                width: 18px;
+                height: 18px;
+                border-radius: 9px;
+            }
+            QCheckBox::indicator:unchecked {
+                border: 2px solid #666666;
+                background: #1d1e22;
+                border-radius: 9px;
+            }
+            QCheckBox::indicator:checked {
+                border: 2px solid #c90000;
+                background: #c90000;
+                border-radius: 9px;
+            }
+            """
+        )
+        app_update_layout.addWidget(self.beta_updates_checkbox)
+        
+        app_update_group.setLayout(app_update_layout)
+        layout.addWidget(app_update_group)
         
         # === yt-dlp Release Channel Section ===
         ytdlp_channel_group = QGroupBox(_("settings.ytdlp_channel"))
@@ -618,6 +652,10 @@ class UpdaterTabWidget(QWidget):
             # Set checkbox
             self.auto_update_enabled.setChecked(auto_settings["enabled"])
             
+            # Load beta setting
+            beta_enabled = ConfigManager.get("check_beta_updates") or False
+            self.beta_updates_checkbox.setChecked(beta_enabled)
+            
             # Set current selection based on saved settings
             current_frequency = auto_settings["frequency"]
             if current_frequency == "startup":
@@ -655,6 +693,10 @@ class UpdaterTabWidget(QWidget):
             frequency = "weekly"
 
         return enabled, frequency
+
+    def get_beta_update_setting(self) -> bool:
+        """Returns the beta update setting from the dialog."""
+        return self.beta_updates_checkbox.isChecked()
     
     def _on_channel_changed(self, checked: bool) -> None:
         """Handle channel selection change."""