Add keychain dialog and settings for unsecure backend usage

- Introduced KeychainDialog for managing keychain access during app store sign-in.
- Added settings for enabling/disabling unsecure backend usage in SettingsManager.
- Updated AppsWidget to initialize keychain dialog based on settings.
- Enhanced error messages and UI adjustments in various components.

src/settingsmanager.h

@@ -55,6 +55,8 @@ public:
     bool unmountiFuseOnExit() const;
     void setUnmountiFuseOnExit(bool enabled);
 #endif
+    bool useUnsecureBackend() const;
+    void setUseUnsecureBackend(bool enabled);
 
     QString theme() const;
     void setTheme(const QString &theme);
@@ -62,6 +64,9 @@ public:
     int connectionTimeout() const;
     void setConnectionTimeout(int seconds);
 
+    bool showKeychainDialog() const;
+    void setShowKeychainDialog(bool show);
+
     // Utility method for conditional execution
     void doIfEnabled(Setting setting, std::function<void()> action);