refactor: adopt POSITIONS constants, fix ESLint warnings, break menuUtils into modules

- Replace hardcoded position strings with POSITIONS.* constants
- Prefix unused variables with _ to resolve no-unused-vars warnings
- Fix no-prototype-builtins with Object.prototype.hasOwnProperty.call()
- Extract menuUtils.js (543 lines) into 6 focused modules under menu/
- menuUtils.js now 35 lines, delegates via prototype mixin pattern
- Add 158 unit tests for ConfigManager, MeasurementContainer, ValidationUtils

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/state/movementManager.js

@@ -216,7 +216,6 @@ class movementManager {
         return reject(new Error("Movement aborted"));
       }
 
-      const direction = targetPosition > this.currentPosition ? 1 : -1;
       const totalDistance = Math.abs(targetPosition - this.currentPosition);
       const startPosition = this.currentPosition;
       this.speed = Math.min(Math.max(this.speed, 0), 1);