CMake: extract data sources into static library, add Catch2 unit tests and CTest integration

[ifilot]

Motivation

• Separate the core data implementation from the GUI target to enable cleaner linking and independent testing of the data layer.
• Add an automated unit test harness to validate data model behavior and loader error handling.
• Integrate Catch2 via FetchContent and enable CTest test discovery to run tests in CI.

Description

• Introduce ATOM_ARCHITECT_DATA_SOURCES and create a new static target atom-architect-data that contains data sources and resources.qrc and links to Qt5::Core, Qt5::Widgets, Qt5::Charts, Eigen3::Eigen, and glm::glm.
• Remove data source files from the atom-architect executable list and link atom-architect against atom-architect-data instead, preserving platform-specific link rules and resource handling.
• Add include(CTest) and a BUILD_TESTING block that fetches Catch2 (v3.7.1), defines atom-architect-tests and wires catch_discover_tests for test discovery.
• Add new unit tests in tests/data_unit_tests.cpp covering Atom selection cycling, Bond construction and special-axis handling, Structure eigenmode validation and selection/freeze behavior, StructureOperator z-alignment, and NebCalculationLoader error reporting.

Testing

• No automated tests were executed as part of this rollout; the repository is configured to build and discover tests when configured with -DBUILD_TESTING=ON.
• The test binary atom-architect-tests is added and linked against Catch2::Catch2WithMain and atom-architect-data for unit execution.
• Tests can be run locally or in CI with cmake -S . -B build -DBUILD_TESTING=ON && cmake --build build && ctest --test-dir build -V.

---

Codex Task