fix(plugins): defensive null checks for settingsSchema and dependents (#197)

The API may not return settingsSchema or dependents fields. Use
optional chaining and nullish coalescing to prevent Object.keys()
and .length crashes on undefined values.

src/components/admin/plugins/plugin-card.tsx

···
         </div>
 
         <div className="flex shrink-0 items-center gap-2">
-          {Object.keys(plugin.settingsSchema).length > 0 && (
+          {Object.keys(plugin.settingsSchema ?? {}).length > 0 && (
             <button
               type="button"
               onClick={() => onOpenSettings(plugin)}

src/components/admin/plugins/plugin-settings-modal.tsx

···
         </div>
 
         <div className="space-y-4">
-          {Object.entries(plugin.settingsSchema).map(([key, schema]) => (
+          {Object.entries(plugin.settingsSchema ?? {}).map(([key, schema]) => (
             <SettingsField
               key={key}
               fieldKey={key}

src/hooks/admin/use-plugin-management.ts

···
   }, [fetchPlugins])
 
   const findDependentNames = (plugin: Plugin): string[] => {
-    return plugin.dependents.map((depId) => {
+    return (plugin.dependents ?? []).map((depId) => {
       const dep = plugins.find((p) => p.id === depId)
       return dep?.displayName ?? depId
     })
   }
 
   const handleToggle = async (plugin: Plugin) => {
-    if (plugin.enabled && plugin.dependents.length > 0) {
+    if (plugin.enabled && (plugin.dependents?.length ?? 0) > 0) {
       const dependentNames = findDependentNames(plugin)
       setDependencyWarning({ plugin, dependents: dependentNames })
       return