Fix thumbnail quality and mobile vertical split layout

Thumbnails: update the actual thumbnail endpoint (ConversionController.js
thumbnailFromBuild) to quality=90 and width=794. The previous fix targeted
ConversionManager.js which handles preview mode, not the thumbnail route
called by ThumbnailManager.mjs.

Mobile layout: move the isMobile guard before the stored-preference check
in getInitialLayout(). The autoSave race fix (build 274) stopped future
bad writes, but a stale 'flat' in localStorage was still being read on
every load, blocking the mobile check. Mobile now always starts in
verticalSplit regardless of any stored value.

CI: add node --check on all server-side .mjs files in the Dockerfile,
after source copy and before webpack compile, so syntax errors like the
escaped-backtick incident fail the build immediately.

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

server-ce/Dockerfile

@@ -54,6 +54,13 @@ RUN --mount=type=cache,target=/root/.cache \
 # Add the actual source files
 # ---------------------------
 COPY --parents libraries/ services/ tools/migrations/ /overleaf/
+
+# Syntax-check all server-side ESM modules before the expensive webpack
+# compile. node --check parses without executing, so it's fast and safe.
+# Catches things like escaped backticks from sed substitutions that webpack
+# never sees (it only bundles frontend code).
+RUN find services/web/app/src services/web/modules -name '*.mjs' | xargs node --check
+
 RUN --mount=type=cache,target=/root/.cache \
     --mount=type=cache,target=/root/.yarn/berry/cache,id=server-ce-yarn-cache \
     --mount=type=tmpfs,target=/usr/local/share/.cache/yarn \

services/clsi/app/js/ConversionController.js

@@ -336,9 +336,9 @@ async function thumbnailFromBuild(req, res) {
       'pdftocairo',
       [
         '-jpeg',
-        '-jpegopt', 'quality=82',
+        '-jpegopt', 'quality=90',
         '-singlefile',
-        '-scale-to-x', '380',
+        '-scale-to-x', '794',
         '-scale-to-y', '-1',
         '-f', '1',
         '-l', '1',

services/web/frontend/js/shared/context/layout-context.tsx

@@ -90,11 +90,13 @@ const MOBILE_MQ = '(max-width: 767px)'
 function getInitialLayout(): IdeLayout {
   const stored = localStorage.getItem('pdf.layout')
   const isMobile = window.matchMedia(MOBILE_MQ).matches
-  // flat is the only preference respected on mobile — both 'split' and
+  // On mobile, always start in verticalSplit (editor above, PDF below).
-  // 'vertical' (which may have been stored from a desktop session) map to
+  // We must check isMobile first: a stale 'flat' in localStorage (written by
-  // verticalSplit because side-by-side is unusable on a narrow screen.
+  // a previous autoSave race or a desktop session) would otherwise short-circuit
-  if (stored === 'flat') return 'flat'
+  // the mobile check and leave the user stuck in a flat/editor-only layout.
+  // The user can still collapse to flat during a session, but it does not persist.
   if (isMobile) return 'verticalSplit'
+  if (stored === 'flat') return 'flat'
   if (stored === 'split') return 'sideBySide'
   if (stored === 'vertical') return 'verticalSplit'
   return 'sideBySide'