feat(gas-profiler): show friendly syncing state when backend nodes are unavailable

Replace generic error alerts with dedicated UI when the proxy returns
503 "currently syncing". Users now see an animated spinner and clear
message instead of a raw error, reducing confusion during node sync.

Author: mattevans

src/pages/ethereum/execution/gas-profiler/SimulatePage.tsx

@@ -105,6 +105,19 @@ function getDeltaColor(percent: number): string {
   return 'text-muted';
 }
 
+/** Detect backend syncing errors (503 from the proxy). */
+function isSyncingError(error: unknown): boolean {
+  if (error instanceof Error) {
+    return error.message.includes('currently syncing');
+  }
+
+  if (typeof error === 'string') {
+    return error.includes('currently syncing');
+  }
+
+  return false;
+}
+
 function calculateAggregateStats(results: BlockSimulationResult[]): AggregateStats {
   const stats = results.reduce(
     (acc, result) => {
@@ -824,14 +837,37 @@ export function SimulatePage(): JSX.Element {
             Loading gas parameters...
           </div>
         )}
-        {defaultsError && (
-          <Alert
-            variant="error"
-            description={`Error loading gas schedule: ${defaultsError.message}`}
-            accentBorder
-            className="mt-3"
-          />
-        )}
+        {defaultsError &&
+          (isSyncingError(defaultsError) ? (
+            <div
+              className="mt-3 overflow-hidden rounded-sm border border-amber-400/30 dark:border-amber-500/20"
+              role="status"
+              aria-live="polite"
+            >
+              <div className="flex items-start gap-3 bg-amber-50/80 p-3.5 dark:bg-amber-500/5">
+                <div className="flex size-8 shrink-0 items-center justify-center rounded-xs bg-amber-100 dark:bg-amber-500/15">
+                  <ArrowPathIcon
+                    className="size-4 animate-spin text-amber-600 dark:text-amber-400"
+                    style={{ animationDuration: '2s' }}
+                  />
+                </div>
+                <div className="min-w-0 flex-1">
+                  <p className="text-sm/5 font-medium text-amber-800 dark:text-amber-200">Backend nodes are syncing</p>
+                  <p className="mt-0.5 text-xs/4 text-amber-700/70 dark:text-amber-300/60">
+                    The nodes serving this network are catching up with the chain. Check back shortly and try again.
+                  </p>
+                </div>
+              </div>
+              <div className="h-px animate-pulse bg-linear-to-r from-transparent via-amber-400/50 to-transparent" />
+            </div>
+          ) : (
+            <Alert
+              variant="error"
+              description={`Error loading gas schedule: ${defaultsError.message}`}
+              accentBorder
+              className="mt-3"
+            />
+          ))}
         {gasWarning && modifiedCount === 0 && (
           <Alert
             variant="warning"
@@ -946,7 +982,37 @@ export function SimulatePage(): JSX.Element {
       {/* Error */}
       {simState.status === 'error' && (
         <div className="mb-4">
-          <Alert variant="error" title="Simulation failed" description={simState.error ?? 'Unknown error'} />
+          {isSyncingError(simState.error) ? (
+            <Card className="overflow-hidden">
+              <div className="flex items-start gap-4 p-5" role="status" aria-live="polite">
+                <div className="relative flex size-10 shrink-0 items-center justify-center">
+                  <div
+                    className="absolute inset-0 animate-ping rounded-full bg-amber-400/15"
+                    style={{ animationDuration: '3s' }}
+                  />
+                  <div className="flex size-10 items-center justify-center rounded-full bg-amber-100 dark:bg-amber-500/10">
+                    <ArrowPathIcon
+                      className="size-5 animate-spin text-amber-500 dark:text-amber-400"
+                      style={{ animationDuration: '2s' }}
+                    />
+                  </div>
+                </div>
+                <div className="min-w-0 flex-1">
+                  <h3 className="text-sm font-semibold text-foreground">Backend Syncing</h3>
+                  <p className="mt-1 text-sm/5 text-muted">
+                    The simulation was interrupted because all backend nodes for this network are currently syncing with
+                    the chain.
+                    {simState.completedBlocks > 0 &&
+                      ` ${simState.completedBlocks} of ${simState.totalBlocks} blocks completed before the interruption.`}{' '}
+                    Check back shortly and try again.
+                  </p>
+                </div>
+              </div>
+              <div className="h-px animate-pulse bg-linear-to-r from-transparent via-amber-400/40 to-transparent" />
+            </Card>
+          ) : (
+            <Alert variant="error" title="Simulation failed" description={simState.error ?? 'Unknown error'} />
+          )}
         </div>
       )}