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.

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

src/UIArc.cpp

@@ -404,7 +404,7 @@ PyGetSetDef UIArc::getsetters[] = {
      "Arc color", NULL},
     {"thickness", (getter)UIArc::get_thickness, (setter)UIArc::set_thickness,
      "Line thickness", NULL},
-    {"click", (getter)UIDrawable::get_click, (setter)UIDrawable::set_click,
+    {"on_click", (getter)UIDrawable::get_click, (setter)UIDrawable::set_click,
      "Callable executed when arc is clicked.", (void*)PyObjectsEnum::UIARC},
     {"z_index", (getter)UIDrawable::get_int, (setter)UIDrawable::set_int,
      "Z-order for rendering (lower values rendered first).", (void*)PyObjectsEnum::UIARC},