feat: add weather formatter module

[baba-dev]

Summary

• add a weather formatter helper that converts Home Assistant climate, sensor, and forecast payloads into Tab5 UI strings and icons
• update the weather page to render sample data through the formatter and respect the preferred temperature unit
• hook the formatter into the unit test suite with new coverage for both Celsius and Fahrenheit paths

Testing

• cmake --build build-tests --target unit_tests
• ctest --test-dir build-tests -R unit_tests --output-on-failure

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https://chatgpt.com/codex/tasks/task_e_68cd5a37511c8324aa44e9d5aed64548