fix: don't resurface missing node warnings on workflow tab switch

When switching between already-opened workflows, the error overlay
for missing nodes was re-shown even after being dismissed. This happened
because openWorkflow always passed showMissingNodes: true, causing
pendingWarnings to be re-set on every loadGraphData call.

Gate showMissingNodes by loadFromRemote, matching the existing pattern
for showMissingModels. Warnings only surface on first load from remote,
not on subsequent tab switches.

src/platform/workflow/core/services/workflowService.ts

@@ -240,7 +240,7 @@ export const useWorkflowService = () => {
       workflow,
       {
         showMissingModels: loadFromRemote,
-        showMissingNodes: true,
+        showMissingNodes: loadFromRemote,
         checkForRerouteMigration: false,
         deferWarnings: true
       }