1. What Has Been Changed

Added #include <element/ui/content.hpp> to include/element/application.hpp to provide the complete definition of ContentFactory before its use with std::unique_ptr.

2. Why It Has Been Changed / Points Addressed

Issue: Build failure on Fedora 43 with GCC 15:

error: invalid application of 'sizeof' to incomplete type 'element::ContentFactory'

Root Cause: The Application::createContentFactory() method returns std::unique_ptr<ContentFactory> initialized with nullptr. When constructing this unique_ptr, the compiler instantiates std::default_delete<ContentFactory>, which requires the complete type definition. Previously, only a forward declaration was available at that point.

Standard Compliance: GCC 15 enforces the C++ standard requirement more strictly than older compilers. The standard requires complete types for std::unique_ptr deleters, though some compilers were more lenient.

3. Test Recommendations / Test Ideas

• Build on GCC 15+ (Fedora 43, Arch with latest GCC) - confirmed
• Build on older GCC versions (GCC 9, 10, 11, 13)
• Build on Clang (Linux, macOS)
• Build on MSVC (Windows)
• Verify all plugin formats still build correctly
• Run existing test suite to confirm no behavioral changes