fix: add flag-icons CSS to admin base template

The language selector component uses flag-icons library for country
flags. This CSS was missing from admin/base.html (already present in
shop and vendor templates).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/templates/admin/base.html

@@ -13,6 +13,9 @@
     <!-- Tailwind CSS v4 (built locally via standalone CLI) -->
     <link rel="stylesheet" href="{{ url_for('static', path='admin/css/tailwind.output.css') }}" />
 
+    <!-- Flag Icons CSS (for language selector) -->
+    <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/flag-icons@7.2.3/css/flag-icons.min.css" />
+
     <!-- Alpine Cloak -->
     <style>
         [x-cloak] { display: none !important; }