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

mise.toml

@@ -13,6 +13,7 @@ all = { run = "go build -buildvcs=false -o fetch . && make -C third_party/akaiut
 serve = { run = "go build -buildvcs=false -o fetch . && ./fetch serve" }
 lint = "golangci-lint run"
 js-lint = "cd web/ui && npm run lint"
+js-test = "cd web/ui && npm test"
 electron-deps = "cd electron && npm install"
 electron-dev = { run = "go build -buildvcs=false -o fetch . && cd electron && npx electron ." }
 electron-build = { run = "go build -buildvcs=false -o fetch . && make -C third_party/akaiutil && cd electron && npx electron-builder --dir" }