Merge Profiles and Session windows into a single window

ProfilesWindow and SessionWindow were two independent top-level windows,
coordinated through a QPointer and manual show/create-or-reuse logic, with
duplicated Help menus and a path where Quit didn't actually quit if a
SessionWindow happened to be open. ProfilesWindow is now an embedded QWidget
shown as a permanent, unclosable "Profiles" tab inside SessionWindow's tab
widget; SessionWindow is the app's sole top-level window. Double-clicking a
profile opens a session tab in the same window instead of finding-or-creating
a second one, and closing all session tabs falls back to the Profiles tab
rather than closing the app.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/session_window.h

@@ -7,23 +7,26 @@
 #include <QMainWindow>
 
 class QTabWidget;
+class ProfilesWindow;
 
 class SessionWindow : public QMainWindow
 {
     Q_OBJECT
 
 public:
-    explicit SessionWindow(const Profile& profile, QWidget* parent = nullptr);
+    explicit SessionWindow(QWidget* parent = nullptr);
     void openProfile(const Profile& profile);
 
 private:
     QTabWidget* m_tabs;
+    ProfilesWindow* m_profilesWidget;
     SessionUiPreferences m_preferences;
 
     void addSessionTab(const Profile& profile);
     void updateTabTitle(SessionTab* tab, const QString& title);
     void loadUiPreferences();
     void saveUiPreferences() const;
+    int sessionTabCount() const;
 };
 
 #endif