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/components/settings/SettingsNav.tsx

@@ -34,16 +34,16 @@ export function SettingsNav({ className }: SettingsNavProps) {
   const isActive = (href: string) => pathname === href || pathname.startsWith(href + '/')
 
   return (
-    <nav className={cn('flex items-center gap-1', className)}>
+    <nav className={cn('flex items-center gap-6 border-b border-border/40', className)}>
       {sections.map((section) => (
         <Link
           key={section.id}
           href={section.href}
           className={cn(
-            'flex items-center gap-2 px-3 py-3 text-sm font-medium border-b-2 transition-colors whitespace-nowrap',
+            'flex items-center gap-2 pb-3 pt-4 text-xs font-semibold uppercase tracking-wider transition-colors whitespace-nowrap border-b-2',
             isActive(section.href)
-              ? 'border-primary text-primary'
-              : 'border-transparent text-muted-foreground hover:text-foreground hover:border-border'
+              ? 'border-primary text-foreground'
+              : 'border-transparent text-muted-foreground hover:text-foreground'
           )}
         >
           {section.icon}