fix: harden backend command validation

[Batuhan4]

Summary

• add shared backend validation helpers for fan IDs, integers, modes, and RGB triplets
• reject malformed fan and keyboard socket input before it reaches helper scripts or sysfs writes
• add a small backend validation test target and update the README to reflect the repo's actual test workflow

Testing

• g++ -std=c++20 -Ibackend/src backend/tests/validation_test.cpp backend/src/validation.cpp -o .tmp-build/backend-validation-test && ./.tmp-build/backend-validation-test
• g++ -std=c++20 -Ibackend/src -c backend/src/validation.cpp -o .tmp-build/validation.o
• g++ -std=c++20 -Ibackend/src -c backend/src/keyboard.cpp -o .tmp-build/keyboard.o
• g++ -std=c++20 -Ibackend/src -c backend/src/main.cpp -o .tmp-build/main.o
• g++ -std=c++20 -Ibackend/src -c backend/src/fan.cpp -o .tmp-build/fan.o

Notes

• Full meson/GTK build validation was not possible in this cloud environment because meson, ninja, and gtk4 development packages are not installed.

  

[github-actions]

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[Batuhan4]

Closing as superseded by the consolidated local fix branch and follow-up testing.