Persist FPS slider and Enter Play Mode dropdown across editor restarts

[alperunlu07]

ToolbarFPSSlider:

• Mark selectedFramerate [SerializeField] (default 60) so the chosen value survives domain reload / editor restart.
• Replace global GUI.changed with EditorGUI.BeginChangeCheck/EndChangeCheck to avoid reacting to unrelated GUI events.
• Save() the toolbar setting on change so toolbar-side edits are persisted even when the Project Settings panel is not open.

ToolbarEnterPlayMode:

• Mark selectedEnterPlayMode [SerializeField] as a defensive cache.
• Lazy-init enterPlayModeOption labels in OnDrawInToolbar in case the initializer hasn't run yet.
• Apply selection to EditorSettings via a dedicated helper, called both on Init() and on user change, so the saved value is enforced on editor open.
• Switch to BeginChangeCheck/EndChangeCheck instead of GUI.changed.
• Save() the toolbar setting on change.