refactor: Rename click property to on_click (closes #139)

Breaking change: callback property standardized to on_* pattern.
- `drawable.click` → `drawable.on_click`

Updated all C++ bindings (8 files) and Python test usages.
Note: src/scripts changes tracked separately (in .gitignore).

This establishes the naming pattern for future callbacks:
on_click, on_enter, on_exit, on_move, on_key, etc.

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

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

src/scripts/game.py

@@ -437,7 +437,7 @@ class SweetButton:
         # box w/ drop shadow
         self.shadow_offset = shadow_offset
         self.base_frame = mcrfpy.Frame(pos=(x, y), size=(box_width+shadow_offset, box_height), fill_color=(0, 0, 0, 255))
-        self.base_frame.click = self.do_click
+        self.base_frame.on_click = self.do_click
 
         # drop shadow won't need configured, append directly
         if shadow:

src/scripts/text_input_widget_improved.py

@@ -127,7 +127,7 @@ class TextInput:
         self.parent_frame.children.append(self.frame)
         
         # Click handler on the input frame
-        self.frame.click = self._on_click
+        self.frame.on_click = self._on_click
     
     def _on_click(self, x, y, button, state):
         """Handle mouse clicks"""