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/util.js

@@ -0,0 +1,29 @@
+// AKAI Utils — shared utility functions
+
+function esc(s) {
+  if (!s) return ''
+  return s.replace(/&/g, '&amp;').replace(/</g, '&lt;')
+          .replace(/>/g, '&gt;').replace(/"/g, '&quot;').replace(/'/g, '&#39;')
+}
+
+function sanitize(s) {
+  return s.replace(/[^a-zA-Z0-9_-]/g, '_')
+}
+
+function formatBytes(bytes) {
+  if (bytes === 0) return '0 B'
+  const k = 1024
+  const sizes = ['B', 'KB', 'MB', 'GB']
+  const i = Math.floor(Math.log(bytes) / Math.log(k))
+  return parseFloat((bytes / Math.pow(k, i)).toFixed(1)) + ' ' + sizes[i]
+}
+
+function formatTime(secs) {
+  const m = Math.floor(secs / 60)
+  const s = Math.floor(secs % 60)
+  return m + ':' + s.toString().padStart(2, '0')
+}
+
+if (typeof module !== 'undefined' && module.exports) {
+  module.exports = { esc, sanitize, formatBytes, formatTime }
+}