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/index.html

@@ -60,6 +60,7 @@
     <div id="search-results"></div>
   </section>
 </div>
-<script src="app.js"></script>
+  <script src="util.js"></script>
+  <script src="app.js"></script>
 </body>
 </html>