refactor: Replace UIIntegration singleton with stateless namespace functions

This commit dismantles the `UIIntegration` singleton class, refactoring its functionality into a set of stateless free functions within the `ui` namespace. This change decouples the UI logic from the lifecycle management of the action handler, resolving ownership ambiguities.

- include/structor/ui_integration.hpp (modified):
  - Removed the `UIIntegration` singleton class entirely.
  - Introduced `namespace ui` to house static UI logic (`initialize`, `shutdown`, `execute_synthesis`).
  - Changed `initialize()` to accept an external `SynthActionHandler*` rather than instantiating one internally.
  - Converted `HexRaysHooks` from a class to a struct.

- src/plugin.cpp (modified):
  - `StructorPlugin` now explicitly owns the `SynthActionHandler` instance as a member variable.
  - Updated initialization and cleanup calls to use the new `ui::` namespace functions.
  - Passes the plugin-owned handler to `ui::initialize`, ensuring the handler's lifetime is bound to the plugin instance (`plugmod_t`).

- include/structor/synth_types.hpp (modified):
  - Removed forward declaration of `UIIntegration`.

Impact:
- No user-visible behavioral changes.
- Improves architectural robustness by ensuring the `SynthActionHandler` is destroyed when the plugin is unloaded, preventing potential use-after-free issues associated with the previous singleton pattern.
- Simplifies the UI integration API by removing unnecessary object state.

Author: 19h

include/structor/synth_types.hpp

@@ -34,7 +34,6 @@ class VTableDetector;
 class TypePropagator;
 class PseudocodeRewriter;
 class StructurePersistence;
-class UIIntegration;
 
 // ============================================================================
 // Core Constants