Fail fast when npm is missing during configure (#14421)

* Warn and skip preview build when npm is missing during configure

Check for npm at the start of configure() instead of failing late in
buildQuartoPreviewJs() after ~300MB of downloads. When npm is missing,
emit a warning explaining that the quarto preview live-reload feature
will not be available, then skip the quarto-preview.js build step.
The rest of Quarto (render, pdf, publish) builds and runs normally.

* Error out instead of skipping when npm is missing

A half-built Quarto without quarto-preview.js would later fail at
runtime when `quarto preview` tries to read the missing file, producing
another confusing error. Requiring npm upfront keeps the contract
simple: configure succeeds only if all required tools are available.

Author: cderv

package/src/common/configure.ts

@@ -18,13 +18,22 @@ import {
   configureDependency,
   kDependencies,
 } from "./dependencies/dependencies.ts";
-import { suggestUserBinPaths } from "../../../src/core/path.ts";
+import { suggestUserBinPaths, which } from "../../../src/core/path.ts";
 import { buildQuartoPreviewJs } from "./previewjs.ts";
 import { isWindows } from "../../../src/deno_ral/platform.ts";
 
 export async function configure(
   config: Configuration,
 ) {
+  // npm is required later for building quarto-preview.js (used by `quarto
+  // preview` live-reload). Check upfront to fail fast before downloading
+  // hundreds of MB of dependencies when npm is missing.
+  if ((await which("npm")) === undefined) {
+    throw new Error(
+      "npm not found on PATH. Please install Node.js (which provides npm) before running configure.",
+    );
+  }
+
   // Download dependencies
   for (const dependency of kDependencies) {
     const targetDir = join(