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/convert/definitions/each.js

@@ -1,5 +1,4 @@
-var metric
-  , imperial;
+var metric;
 
 metric = {
   ea: {

src/convert/definitions/partsPer.js

@@ -1,5 +1,4 @@
-var metric
-  , imperial;
+var metric;
 
 metric = {
   ppm: {