Code audit hardening: backend input validation and frontend error handling

[Batuhan4]

Summary

• replace shell-constructed backend helper execution with argument-safe fork/exec invocation
• reject invalid fan ids and malformed fan speed payloads at the backend command boundary
• harden backend keyboard handling by validating RGB triplets and clamping brightness writes to 0..255
• fix frontend fatal-error dialog path so the temporary loop exits cleanly instead of hanging
• improve frontend preset home-directory resolution when $HOME is unset

Verification

• g++ -std=c++20 -Wall -Wextra -pedantic backend/src/main.cpp backend/src/fan.cpp backend/src/keyboard.cpp backend/src/util.cpp -o /tmp/victus-backend-test -pthread
• bash -n install.sh arch-install.sh fedora-install.sh backend/src/set-fan-speed.sh backend/src/set-fan-mode.sh backend/src/set-rgb-zone.sh backend/victus-healthcheck.sh

Notes

• Full Meson build could not run in this environment because meson and GTK4 dev packages are unavailable (meson: command not found, pkg-config gtk4 not found).

  

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[Batuhan4]

Closing as superseded by the consolidated local fix branch and follow-up testing.