Add runtime MainMenu injection and docs

Introduce a C++ runtime injection that adds a Multiplayer row to the Fallout 4 main menu instead of patching MainMenu.swf. Adds F4TMainMenuInject.h/cpp implementing injection, ITEM_PRESS handling, accept/button hooks, resync tasks and logging; integrates installation by calling MainMenuInject::Install() from PrismaUI init. Update PrismaUI to use the new helper and remove a duplicate IsMainMenuOnMainPanel implementation. Documentation updated to deprecate FFDec SWF patching (new docs/MainMenu_Injection.md, note in FFDEC guide, and setup.md) and restore-vanilla-mainmenu.ps1 messaging updated to instruct using the runtime injection.

plugin/include/F4TMainMenuInject.h

@@ -0,0 +1,12 @@
+#pragma once
+
+namespace RE
+{
+	class UI;
+}
+
+namespace F4T::MainMenuInject
+{
+	bool IsMainMenuOnMainPanel(const RE::UI* a_ui);
+	void Install();
+}