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>

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
-  // 'vertical' (which may have been stored from a desktop session) map to
-  // verticalSplit because side-by-side is unusable on a narrow screen.
-  if (stored === 'flat') return 'flat'
+  // On mobile, always start in verticalSplit (editor above, PDF below).
+  // We must check isMobile first: a stale 'flat' in localStorage (written by
+  // a previous autoSave race or a desktop session) would otherwise short-circuit
+  // 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'