[#114.3] Add iwyu enforcement to intermediate examples

[thomasnemer]

Summary

Add CXX_INCLUDE_WHAT_YOU_USE property to all 4 intermediate examples (01_nested_panels, 02_responsive_ui, 03_event_handling, 04_dynamic_layout) and fix include issues identified by iwyu.

This is part of #114 - making include-what-you-use mandatory for all example builds.

Changes

CMakeLists.txt modifications

• Add iwyu enforcement to all 4 intermediate example targets
• Use configuration: "${IWYU_PATH};-Xiwyu;--error;-Xiwyu;--verbose=3"

Include fixes

• 01_nested_panels/main.cpp: Added component_traits.h for Color type
• 02_responsive_ui/main.cpp: Added theming/color.h for theming::Color
• 03_event_handling/main.cpp: Added theming/color.h, removed unused Button include
• 04_dynamic_layout/main.cpp: Added component_traits.h and <vector>, added forward declaration

Test Plan

• mise build-examples succeeds without iwyu errors
• All 55 targets build successfully
• Git hooks (format-check, build-validation) pass
• All intermediate examples now enforce include-what-you-use

Related

• Closes [#114.3] Add iwyu enforcement to intermediate examples #119
• Part of Make include-what-you-use mandatory for all example builds #114
• Follows [#114.1] Add iwyu enforcement to glfw_adapters and demo_app #121 (Make include-what-you-use mandatory for all example builds #114.1 - glfw_adapters and demo_app)
• Follows [#114.2] Add iwyu enforcement to basic examples #122 (Make include-what-you-use mandatory for all example builds #114.2 - basic examples)