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/general/general-settings-client.tsx

@@ -50,11 +50,9 @@ export function GeneralSettingsClient({ initialSettings }: GeneralSettingsClient
 
   return (
     <div className="space-y-8">
-      {/* Page title */}
-      <div>
-        <h1 className="text-2xl font-bold tracking-tight text-foreground">{t('generalSettings.title')}</h1>
-        <p className="text-muted-foreground mt-1">{t('generalSettings.description')}</p>
-      </div>
+      <p className="text-[11px] font-bold uppercase tracking-[0.2em] text-muted-foreground">
+        {t('generalSettings.description')}
+      </p>
 
       {/* 2-column card grid */}
       <div className="grid grid-cols-1 md:grid-cols-2 gap-6">