feat: show persisted sync errors

scripts/check.sh

@@ -223,6 +223,8 @@ if command -v node &>/dev/null; then
   report "browser inbox frontend behavior" $?
   node "$ROOT/scripts/smoke-search-plugin.js"
   report "search frontend behavior" $?
+  node "$ROOT/scripts/smoke-sync-plugin.js"
+  report "sync frontend behavior" $?
 else
   echo "  ⚠️  node not available — skipping frontend smoke"
 fi

scripts/smoke-sync-plugin.js

@@ -0,0 +1,19 @@
+#!/usr/bin/env node
+const fs = require('fs');
+const path = require('path');
+
+const root = path.resolve(__dirname, '..');
+const sourcePath = path.join(root, 'plugins', 'sync', 'frontend', 'src', 'SyncSettings.svelte');
+const source = fs.readFileSync(sourcePath, 'utf8');
+
+if (!source.includes('settings.lastError')) {
+  throw new Error('SyncSettings must render persisted settings.lastError');
+}
+if (!source.includes('sanitizeError(settings.lastError)')) {
+  throw new Error('SyncSettings must sanitize persisted sync errors before rendering');
+}
+if (!source.includes('Last sync error')) {
+  throw new Error('SyncSettings must label the persisted sync error');
+}
+
+console.log('sync plugin smoke passed');