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/ToggleSwitch.tsx

@@ -0,0 +1,18 @@
+import React from 'react'
+
+export default function ToggleSwitch({ on, onToggle }: { on: boolean; onToggle: () => void }) {
+  return (
+    <button onClick={onToggle}
+      style={{
+        position: 'relative', width: 44, height: 24, borderRadius: 12, border: 'none', cursor: 'pointer',
+        background: on ? 'var(--accent, #111827)' : 'var(--border-primary, #d1d5db)',
+        transition: 'background 0.2s',
+      }}>
+      <span style={{
+        position: 'absolute', top: 2, left: on ? 22 : 2,
+        width: 20, height: 20, borderRadius: '50%', background: 'white',
+        transition: 'left 0.2s', boxShadow: '0 1px 3px rgba(0,0,0,0.2)',
+      }} />
+    </button>
+  )
+}