fix(pfe-tools): redirect .js to .ts only on 404

The previous approach (serve hook) broke esbuild compilation because
returning from serve() prevents other plugins from running. Instead,
keep the koa middleware but call next() first, then redirect to .ts
only when the .js file doesn't exist (404). This is backwards
compatible: when compiled .js exists, it serves normally.

Assisted-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: bennypowers

tools/pfe-tools/dev-server/config.ts

@@ -1,4 +1,4 @@
-import type { Plugin, Context } from '@web/dev-server-core';
+import type { Plugin, Context, Middleware } from '@web/dev-server-core';
 import type { DevServerConfig } from '@web/dev-server';
 
 import { readdir, stat } from 'node:fs/promises';
@@ -85,19 +85,17 @@ async function cacheBusterMiddleware(ctx: Context, next: () => Promise<any>) {
   }
 }
 
-function tsRedirectPlugin(
+function liveReloadTsChangesMiddleware(
   config: ReturnType<typeof normalizeOptions>,
-): Plugin {
-  const TYPESCRIPT_SOURCES_RE = new RegExp(`(${config.elementsDir}|pfe-core)/.*\\.js$`);
-  return {
-    name: 'pfe-ts-redirect',
-    serve(context) {
-      if (!context.path.includes('node_modules')
-          && TYPESCRIPT_SOURCES_RE.test(context.path)) {
-        // Rewrite .js to .ts so esbuild plugin can compile it
-        context.path = context.path.replace(/\.js$/, '.ts');
-      }
-    },
+): Middleware {
+  const TYPESCRIPT_SOURCES_RE = new RegExp(`(${config.elementsDir}|pfe-core)/.*\\.js`);
+  return async function(ctx, next) {
+    await next();
+    if (ctx.status === 404
+        && !ctx.path.includes('node_modules')
+        && TYPESCRIPT_SOURCES_RE.test(ctx.path)) {
+      ctx.redirect(ctx.path.replace('.js', '.ts'));
+    }
   };
 }
 
@@ -118,13 +116,11 @@ export function pfeDevServerConfig(options?: PfeDevServerConfigOptions): DevServ
     middleware: [
       cors,
       cacheBusterMiddleware,
+      liveReloadTsChangesMiddleware(config),
       ...config?.middleware ?? [],
     ],
 
     plugins: [
-      // Rewrite .js paths to .ts before static file serving
-      tsRedirectPlugin(config),
-
       // Dev server app which loads component demo files
       pfeDevServerPlugin(config),