Fail vite build on rolldown warnings via NODE_ENV=test (#37270)

Fail the vite build on any rolldown warnings when `NODE_ENV=test` is
set. This gate is set on the CI `make frontend` steps (compliance and
e2e workflows) and on the local `make test-e2e` target, so warnings fail
the build both in CI and when running e2e tests locally. Regular `make
frontend` / production builds are unaffected.

Example output:

```
[plugin test-warning-injector] first synthetic warning
[plugin test-warning-injector] second synthetic warning
transforming...✗ Build failed in 14ms
error during build:
Build failed with 1 error:

[plugin fail-on-warnings]
Error: 2 warnings present
    at PluginContextImpl.buildEnd (vite.config.ts:50:13)
    ...
```

---
This PR was written with the help of Claude Opus 4.7

---------

Signed-off-by: silverwind <me@silverwind.io>
Co-authored-by: Claude (Opus 4.7) <noreply@anthropic.com>
Co-authored-by: Nicolas <bircni@icloud.com>

vite.config.ts

@@ -38,10 +38,25 @@ const webComponents = new Set([
   'text-expander',
 ]);
 
+function failOnWarningsPlugin(): Rolldown.Plugin {
+  let warningCount = 0;
+  return {
+    name: 'fail-on-warnings',
+    onLog(level) {
+      if (level === 'warn') warningCount++;
+    },
+    buildEnd() {
+      if (!warningCount) return;
+      throw new Error(`${warningCount} warnings present`);
+    },
+  };
+}
+
 const commonRolldownOptions: Rolldown.RolldownOptions = {
   checks: {
     pluginTimings: false,
   },
+  ...(env.CI ? {plugins: [failOnWarningsPlugin()]} : {}),
 };
 
 function commonViteOpts({build, ...other}: InlineConfig): InlineConfig {