Warn about missing images/videos in Quarto HTML output

Since we dropped --embed-resources (so RevealJS plugins like chalkboard work),
pandoc no longer tries to fetch referenced media for HTML output, so a missing
image or video produces no compile-time warning — it only renders broken in the
browser. PDF/Typst output is unaffected because Typst hard-errors on a missing
image.

After an HTML render, QuartoRunner now scans output.html for local media
references (img/video/audio/iframe src, poster, RevealJS data-background-*) and
appends a `[WARNING] Missing resource: …` line to output.log for any that don't
exist on disk. External URLs, data URIs, anchors and Quarto's own generated
<basename>_files assets are ignored. The [WARNING] prefix is recognised by the
Quarto/Typst log parser, so these show up in the Warnings tab.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

services/clsi/app/js/QuartoRunner.js

@@ -42,7 +42,9 @@ function runQuarto(compileName, options, callback) {
       // error object; merge it so _writeLogOutput can persist it.
       const combined = output || (error ? { stdout: error.stdout || '' } : null)
       _writeLogOutput(compileName, directory, combined, () =>
-        callback(null, combined)
+        _appendMissingResourceWarnings(directory, () =>
+          callback(null, combined)
+        )
       )
     }
   )
@@ -96,6 +98,65 @@ function _writeLogOutput(compileName, directory, output, callback) {
   })
 }
 
+// Quarto's HTML/RevealJS output is NOT self-contained (we deliberately dropped
+// --embed-resources so reveal plugins like chalkboard work). A side effect is
+// that pandoc no longer tries to fetch referenced media, so a missing image or
+// video produces no compile-time warning — it just renders broken in the
+// browser. To restore that feedback, scan the produced output.html for local
+// media references and emit a [WARNING] for any that don't exist on disk. The
+// [WARNING] prefix is understood by the Quarto/Typst log parser on the web
+// side, so these surface in the Warnings tab like any other.
+//
+// Only HTML output is scanned: PDF output (Typst) already hard-errors on a
+// missing image, so it needs no extra check.
+function _appendMissingResourceWarnings(directory, callback) {
+  const htmlFile = Path.join(directory, 'output.html')
+  fs.readFile(htmlFile, 'utf8', (err, html) => {
+    if (err) return callback() // no HTML output (e.g. a PDF compile)
+
+    const missing = _extractLocalMediaRefs(html).filter(ref => {
+      try {
+        return !fs.existsSync(Path.join(directory, decodeURIComponent(ref)))
+      } catch {
+        return false
+      }
+    })
+    if (missing.length === 0) return callback()
+
+    const warnings =
+      missing
+        .map(
+          ref =>
+            `[WARNING] Missing resource: ${ref} (referenced in the document ` +
+            `but not found in the project — it will appear broken)`
+        )
+        .join('\n') + '\n'
+    fs.appendFile(Path.join(directory, 'output.log'), '\n' + warnings, () =>
+      callback()
+    )
+  })
+}
+
+// Pull local media references (img/video/audio/iframe src, poster, RevealJS
+// data-background-*) out of the rendered HTML. External URLs, data URIs and
+// in-page anchors are ignored; Quarto's own generated assets (under
+// <basename>_files/) exist on disk, so they never get flagged.
+function _extractLocalMediaRefs(html) {
+  const refs = new Set()
+  const attrRegex =
+    /(?:src|poster|data-background-image|data-background-video)\s*=\s*["']([^"']+)["']/gi
+  let match
+  while ((match = attrRegex.exec(html)) !== null) {
+    const url = match[1].trim()
+    if (!url) continue
+    // Skip absolute URLs, protocol-relative, data/blob URIs and anchors.
+    if (/^(?:[a-z]+:|\/\/|\/|#|data:|blob:)/i.test(url)) continue
+    const clean = url.split(/[?#]/)[0] // drop query string / fragment
+    if (clean) refs.add(clean)
+  }
+  return [...refs]
+}
+
 function isRunning(compileName) {
   return ProcessTable[compileName] != null
 }