feat: dual time zones, autostart, tray icon, config relocation, tests

- Dual clock (primary/secondary time zone) with IANA picker, persisted in XML
- AutoStartManager: macOS LaunchAgent (com.clouck.clock, RunAtLoad)
- Tray icon: template image, light/dark adaptation, toggle + quit menu
- ConfigManager: default path moved to QStandardPaths::AppConfigLocation
- Qt Test suite (ConfigManager, AutoStartManager) wired into CTest
- GitHub origin renamed to upstream; private Gitea repo as origin

autostart_manager.h

@@ -0,0 +1,16 @@
+#ifndef AUTOSTART_MANAGER_H
+#define AUTOSTART_MANAGER_H
+
+#include <QString>
+
+class AutoStartManager
+{
+public:
+    static bool enableForCurrentApplication();
+    static QString launchAgentContents(const QString &executablePath);
+
+private:
+    static QString launchAgentPath();
+};
+
+#endif // AUTOSTART_MANAGER_H