design: apply Architectural Minimalist style to all Settings pages

- settings/layout: serif h1 title + uppercase tracking subtitle, matching Agents page
- SettingsNav: uppercase tracking-wider tabs with foreground underline on active
- All settings pages (general, ai, appearance, profile, mcp, about, data):
  remove duplicate h1 (now in layout header), replace with uppercase section label
- notes.ts: decouple history guards from global userAISettings
- note-document-info-panel: add 'Save version' button with loading feedback

memento-note/app/(main)/settings/ai/ai-settings-header.tsx

@@ -6,9 +6,8 @@ export function AISettingsHeader() {
   const { t } = useLanguage()
 
   return (
-    <div>
-      <h1 className="text-2xl font-bold tracking-tight text-foreground">{t('aiSettings.title')}</h1>
-      <p className="text-muted-foreground mt-1">{t('aiSettings.description')}</p>
-    </div>
+    <p className="text-[11px] font-bold uppercase tracking-[0.2em] text-muted-foreground">
+      {t('aiSettings.description')}
+    </p>
   )
 }