refactor: split large files into single-responsibility modules, add server-configuration section

Frontend:
- Extract Layout.tsx (801 lines) into navData.ts, Sidebar.tsx, and thin Layout.tsx
- Split Settings.tsx into settings/FormControls, ContactTab, BasicTab, NameserversSection
- Add server-configuration/ section with index card grid and one file per tool (13 tools)
- Add shared SectionCard and ComingSoon components
- Nav category headers with a section index link (optional `to` field on NavCategory)
- /settings redirects to /server-configuration/basic-setup

Backend:
- Split accounts.ts (671 lines) into accounts/list, create, modify, status sub-plugins
- Add GET /api/nixserver/system/network-interfaces for ETHDEV dropdown

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

nixserver/src/pages/server-configuration/UpdatePreferences.tsx

@@ -0,0 +1,10 @@
+import ComingSoon from '../../components/ComingSoon'
+
+export default function UpdatePreferences() {
+  return (
+    <ComingSoon
+      title="Update Preferences"
+      description="Control automatic update behaviour, channels, and scheduling."
+    />
+  )
+}