feat(settings): remake settings page with admin-style tabbed layout

Replaces the 2-column masonry layout with a horizontal pill tab bar
matching the admin page pattern. Extracts all sections into self-contained
components under components/Settings/ and reduces SettingsPage.tsx from
1554 lines to 93. Adds i18n tab label keys across all 13 language files.

client/src/components/Settings/Section.tsx

@@ -0,0 +1,22 @@
+import React from 'react'
+import type { LucideIcon } from 'lucide-react'
+
+interface SectionProps {
+  title: string
+  icon: LucideIcon
+  children: React.ReactNode
+}
+
+export default function Section({ title, icon: Icon, children }: SectionProps): React.ReactElement {
+  return (
+    <div className="rounded-xl border overflow-hidden" style={{ background: 'var(--bg-card)', borderColor: 'var(--border-primary)', marginBottom: 24 }}>
+      <div className="px-6 py-4 border-b flex items-center gap-2" style={{ borderColor: 'var(--border-secondary)' }}>
+        <Icon className="w-5 h-5" style={{ color: 'var(--text-secondary)' }} />
+        <h2 className="font-semibold" style={{ color: 'var(--text-primary)' }}>{title}</h2>
+      </div>
+      <div className="p-6 space-y-4">
+        {children}
+      </div>
+    </div>
+  )
+}