feat: Phase 1 - safe constructors and _Drawable foundation

Closes #7 - Make all UI class constructors safe:
- Added safe default constructors for UISprite, UIGrid, UIEntity, UICaption
- Initialize all members to predictable values
- Made Python init functions accept no arguments
- Added x,y properties to UIEntity

Closes #71 - Create _Drawable Python base class:
- Created PyDrawable.h/cpp with base type (not yet inherited by UI types)
- Registered in module initialization

Closes #87 - Add visible property:
- Added bool visible=true to UIDrawable base class
- All render methods check visibility before drawing

Closes #88 - Add opacity property:
- Added float opacity=1.0 to UIDrawable base class
- UICaption and UISprite apply opacity to alpha channel

Closes #89 - Add get_bounds() method:
- Virtual method returns sf::FloatRect(x,y,w,h)
- Implemented in Frame, Caption, Sprite, Grid

Closes #98 - Add move() and resize() methods:
- move(dx,dy) for relative movement
- resize(w,h) for absolute sizing
- Caption resize is no-op (size controlled by font)

🤖 Generated with [Claude Code](https://claude.ai/code)

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

src/UIGrid.h

@@ -34,6 +34,11 @@ public:
     PyObjectsEnum derived_type() override final;
     //void setSprite(int);
     virtual UIDrawable* click_at(sf::Vector2f point) override final;
+    
+    // Phase 1 virtual method implementations
+    sf::FloatRect get_bounds() const override;
+    void move(float dx, float dy) override;
+    void resize(float w, float h) override;
 
     int grid_x, grid_y;
     //int grid_size; // grid sizes are implied by IndexTexture now