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/helper/menu/index.js

@@ -0,0 +1,18 @@
+/**
+ * menu/index.js
+ * Barrel file for the menu module components.
+ */
+
+const toggles = require('./toggles');
+const dataFetching = require('./dataFetching');
+const urlUtils = require('./urlUtils');
+const dropdownPopulation = require('./dropdownPopulation');
+const htmlGeneration = require('./htmlGeneration');
+
+module.exports = {
+  toggles,
+  dataFetching,
+  urlUtils,
+  dropdownPopulation,
+  htmlGeneration,
+};