fix: drop useless string concat in devTools KEY to unblock CI build

react-scripts build with CI=true treats the no-useless-concat ESLint
warning as an error. DefinePlugin inlining is already prevented by the
dynamic process.env[KEY] lookup, so the literal split was unnecessary.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/config/devTools.js

@@ -1,6 +1,7 @@
 // Dev-tools gate. Explicit opt-in via REACT_APP_DEV_TOOLS=1.
 // Safe default: any value other than exactly '1' = off.
-// Dynamic key access so react-scripts DefinePlugin does NOT inline the value
-// at build time — allows tests + runtime env changes to take effect.
-const KEY = 'REACT_APP_' + 'DEV_TOOLS';
+// Dynamic key access (process.env[KEY]) so react-scripts DefinePlugin does NOT
+// inline the value at build time — allows tests + runtime env changes to take
+// effect. DefinePlugin only replaces static member access, never computed lookups.
+const KEY = 'REACT_APP_DEV_TOOLS';
 export const isDevToolsEnabled = () => process.env[KEY] === '1';