feat: generate first-slide thumbnail for Quarto RevealJS presentations

thumbnailFromBuild() now tries output.pdf → output-slides.pdf → decktape
on output.html (slide 1 only). The web service's ThumbnailManager already
calls this endpoint fire-and-forget on every successful compile, so RevealJS
project cards will show the first slide thumbnail automatically.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

services/clsi/app/js/ConversionController.js

@@ -256,26 +256,73 @@ async function convertProjectToDocument(req, res) {
   }
 }
 
-// Generates a JPEG thumbnail of page 1 of the compiled output.pdf using
+// Generates a JPEG thumbnail of page 1 of the compiled output using
-// pdftocairo (poppler-utils, installed in the CLSI base image). Does NOT
+// pdftocairo (poppler-utils). Tries output.pdf first (LaTeX / Quarto-PDF),
-// require ENABLE_PDF_CONVERSIONS or the pdftocairo Docker image.
+// then output-slides.pdf (Quarto RevealJS after a PDF-export compile), then
+// falls back to rendering slide 1 of output.html via decktape (normal
+// RevealJS preview compile). All temp dirs are cleaned up in finally.
 async function thumbnailFromBuild(req, res) {
   const { project_id: projectId, user_id: userId, build_id: buildId } = req.params
   if (!buildId?.match(OutputCacheManager.BUILD_REGEX)) return res.sendStatus(400)
 
   const compileName = userId ? `${projectId}-${userId}` : projectId
-  const pdfPath = Path.join(
+  const buildDir = Path.join(
     Settings.path.outputDir,
     compileName,
     OutputCacheManager.CACHE_SUBDIR,
-    buildId,
+    buildId
-    'output.pdf'
   )
 
-  try {
+  let pdfPath = null
-    await fs.access(pdfPath)
+  let deckTapeDir = null
-  } catch {
+
-    return res.sendStatus(404)
+  for (const name of ['output.pdf', 'output-slides.pdf']) {
+    try {
+      const p = Path.join(buildDir, name)
+      await fs.access(p)
+      pdfPath = p
+      break
+    } catch {}
+  }
+
+  if (!pdfPath) {
+    const htmlPath = Path.join(buildDir, 'output.html')
+    try {
+      await fs.access(htmlPath)
+      deckTapeDir = await fs.mkdtemp(Path.join(os.tmpdir(), 'clsi-deck-'))
+      const chromeHome = Path.join(deckTapeDir, 'chrome')
+      await fs.mkdir(chromeHome, { recursive: true })
+      const slidePdf = Path.join(deckTapeDir, 'slide1.pdf')
+      await execFileAsync(
+        'decktape',
+        [
+          '--slides', '1',
+          '--chrome-arg=--no-sandbox',
+          '--chrome-arg=--disable-dev-shm-usage',
+          '--chrome-arg=--disable-gpu',
+          `--chrome-arg=--user-data-dir=${chromeHome}/data`,
+          htmlPath,
+          slidePdf,
+        ],
+        {
+          timeout: 60000,
+          env: {
+            ...process.env,
+            HOME: chromeHome,
+            XDG_CONFIG_HOME: chromeHome,
+            XDG_CACHE_HOME: chromeHome,
+          },
+        }
+      )
+      pdfPath = slidePdf
+    } catch (err) {
+      logger.warn({ err, projectId, buildId }, 'decktape slide1 thumbnail failed')
+      if (deckTapeDir) {
+        await fs.rm(deckTapeDir, { recursive: true, force: true }).catch(() => {})
+        deckTapeDir = null
+      }
+      return res.sendStatus(404)
+    }
   }
 
   const tmpDir = await fs.mkdtemp(Path.join(os.tmpdir(), 'clsi-thumb-'))
@@ -311,6 +358,9 @@ async function thumbnailFromBuild(req, res) {
     if (!res.headersSent) res.sendStatus(500)
   } finally {
     await fs.rm(tmpDir, { recursive: true, force: true }).catch(() => {})
+    if (deckTapeDir) {
+      await fs.rm(deckTapeDir, { recursive: true, force: true }).catch(() => {})
+    }
   }
 }