fix(useColorModes): clean up the matchMedia listener#510
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The prefers-color-scheme change listener was added inside a useEffect with no dependency array and no cleanup, so a new listener was registered on every render and none were ever removed (a leak). Extract the handler, return a cleanup that removes it, and scope the effect to [colorMode, localStorageItemName].
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Fixes a listener leak from the July 2026 security/quality audit (P2).
The
prefers-color-schemematchMediachange listener was added inside auseEffectwith no dependency array and no cleanup — so a new listener was registered on every render and none were ever removed. Extract the handler, return a cleanup that removes it, and scope the effect to[colorMode, localStorageItemName]. Regression tests: the listener is removed on unmount, and re-renders don't stack new listeners.Local validation (GitHub CI unavailable — billing): lint 0 errors, lib:build clean, lib:test green (ran via the pre-push gate).