fix: MindSpace remote sync, thumbnail fallback, and Plaza URL guards

Ensure page sync runs via pageSyncService in remote mode, fall back to
workspace HTML when storage assets are missing, and prevent production
Portal from linking to loopback Plaza URLs baked in at build time.

Co-authored-by: Cursor <cursoragent@cursor.com>

mindspace-server-adapter-contract.mjs

@@ -44,6 +44,7 @@ export const MINDSPACE_SERVER_ADAPTER_BINDINGS = Object.freeze({
     'updatePage',
     'uploadThumbnail',
   ]),
+  pageSyncService: Object.freeze(['syncUserGeneratedPages']),
   pageLiveEditService: Object.freeze(['applyAgentPatch', 'bindSession', 'getRevisionSnapshot']),
   assetAgentService: Object.freeze(['applyAgentDelete']),
   publicationService: Object.freeze([