Refactor large modules into focused submodules

[RAprogramm]

Summary

• split the AppCode enum implementation into a dedicated src/code/app_code.rs module while keeping the public re-export small
• break the frontend WebAssembly helpers into focused error, extension, and test modules to keep files concise
• decompose the Turnkey integration into domain, classifier, conversion, and test modules for better organization

Testing

• cargo +nightly fmt --
• cargo build --all-targets
• cargo test --all
• cargo clippy -- -D warnings
• cargo doc --no-deps

---

https://chatgpt.com/codex/tasks/task_e_68ccacdde0c4832b8b4eed372b8b1e6e