feat(gui): add OriginManagerDialog for managing file origins (#15)

- Create OriginManagerDialog with list of configured origins
- Show connection status for remote origins
- Allow setting default origin
- Add/Edit/Remove buttons (Add/Edit show placeholder for now)
- Wire up 'Manage Origins...' button in OriginSelectorWidget
- Prevent removal of built-in local origin

Part of Issue #15: Multi-instance Silo configuration UI

src/Gui/OriginSelectorWidget.cpp

@@ -26,6 +26,7 @@
 
 #include "OriginSelectorWidget.h"
 #include "OriginManager.h"
+#include "OriginManagerDialog.h"
 #include "FileOrigin.h"
 #include "BitmapFactory.h"
 
@@ -151,8 +152,12 @@ void OriginSelectorWidget::onOriginActionTriggered(QAction* action)
 
 void OriginSelectorWidget::onManageOriginsClicked()
 {
-    // TODO: Open origins management dialog (Issue #15)
-    // For now, this is a placeholder
+    OriginManagerDialog dialog(this);
+    dialog.exec();
+
+    // Refresh the menu in case origins changed
+    rebuildMenu();
+    updateDisplay();
 }
 
 void OriginSelectorWidget::updateDisplay()