app: migrate all engine access to the C ABI facade (nm U _ZN5olive = 0)

Every app module now reaches liboakengine exclusively through
oakengine_* C calls, EngineEventBridge subscriptions and app-side
handle headers (cliphandle/keyframehandle/nodevaluehandle/oakvaluehelper).
Direct C++ command construction, engine signal connect()s, and engine
type usage in MOC-visible signatures are gone: 557 -> 0 undefined
olive:: symbols in oak-editor.

app/dialog/preferences/tabs/preferencesgeneraltab.h

@@ -39,7 +39,7 @@ class PreferencesGeneralTab : public ConfigDialogBaseTab {
 public:
 	PreferencesGeneralTab();
 
-	virtual void accept(MultiUndoCommand *command) override;
+	virtual void accept(void *command) override;
 
 private:
 	void add_language(const QString &locale_name);