test: add JS test framework using Node built-in test runner

Extracted pure utility functions (esc, sanitize, formatBytes, formatTime)
into web/ui/util.js with CommonJS export for Node testing compatibility.
Added 19 tests across 4 suites using node:test + node:assert (zero deps).

- util.js: browser-compatible utility functions
- util.test.js: 19 tests (5 esc, 3 sanitize, 6 formatBytes, 5 formatTime)
- package.json: 'test' script → node --test util.test.js
- mise.toml: js-test task
- app.js: removed duplicate helper functions (now in util.js)
- index.html: load util.js before app.js

web/ui/package.json

@@ -3,7 +3,8 @@
   "version": "1.0.0",
   "description": "Web UI for AKAI Utils",
   "scripts": {
-    "lint": "eslint *.js"
+    "lint": "eslint *.js",
+    "test": "node --test util.test.js"
   },
   "devDependencies": {
     "eslint": "^9.0.0"