Move Settings::Main class to anonymous namespace

- Move Main class definition from header to .cpp file
- Export only Type MainId() from header
- Update all external references from Main::Id() to MainId()
- Remove Builder::MainSection export from namespace
- Add missing includes for settings_common_session.h where needed

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Telegram/SourceFiles/window/window_session_controller.cpp

@@ -1460,7 +1460,7 @@ void SessionNavigation::showSettings(
 }
 
 void SessionNavigation::showSettings(const SectionShow &params) {
-	showSettings(Settings::Main::Id(), params);
+	showSettings(Settings::MainId(), params);
 }
 
 void SessionNavigation::showPollResults(