CI: nothing checks that the web dashboard's JavaScript modules parse #4672

Nothing in CI checks that the Darling web dashboard's JavaScript modules (Darling/PerformanceMonitor.Darling.Service/wwwroot/js/**/*.js) parse.

• How they're tested today: the C# tests pin them by reading their source text, and no step runs or parses them.
• The result: a module with a syntax error passes every check. In a browser, one syntax error in app.js stops the whole dashboard from loading.

Seen

• What happened: during work on Web dashboard auto-refresh: per-view/notebook refresh setting (notebooks default off), a master pause, and back-off for slow pages (user feedback) #4666, a branch's app.js briefly contained about 220 duplicated lines: a function body without its header, and second copies of several top-level functions.
• The error: node --check reports SyntaxError: Illegal return statement.
• What CI would have said: the branch's C# tests all passed (725), so nothing in CI would have caught it. It was found by hand before a PR was opened.

Expected

• A CI step that parses every wwwroot/js file as an ES module, for example node --check on each file, or an import() of each under a stub DOM.
• It fails the PR on any syntax error.
• When it runs: on any change under wwwroot/js, while a docs-only change still takes the docs fast path.