Reorganize source code into src/ directory

[Claude]

Moves all C++ source files (.cpp, .h, .mm) and CMake template files into a dedicated src/ directory for better project organization.

Changes

• Source organization: All 24 source files relocated to src/ using git mv to preserve history
• Build configuration: Updated CMakeLists.txt to reference src/ paths in:
  • Template file generation (version.h.in, UpdateConfig.h.in)
  • All three platform-specific executable definitions (macOS, Windows, Linux)
  • Source file properties (Objective-C++ compilation flags)
• Documentation: Updated README.md and AGENTS.md with comprehensive module listings and new structure

Repository structure

Phraims/
├── src/                    # All C++ source code
│   ├── main.cpp
│   ├── SplitWindow.h/.cpp
│   ├── SplitFrameWidget.h/.cpp
│   ├── UpdateChecker.h/.cpp
│   └── ... (24 files total)
├── resources/              # Icons, Qt resources, templates
├── ci/                     # CI/CD build scripts
└── CMakeLists.txt

GitHub Actions workflows require no changes—they invoke CMake which now references the updated paths.