feat: add local fallback for flag-icons CSS

- Download flag-icons CSS and SVG sprites for supported languages (gb, fr, de, lu)
- Add onerror fallback to all base templates (admin, vendor, shop)
- CDN loads first, falls back to local copy if unavailable

This ensures the language selector flags work even when CDN is blocked
or unavailable.

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

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

app/templates/admin/base.html

@@ -13,8 +13,12 @@
     <!-- 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" />
+    <!-- Flag Icons CSS (for language selector) with local fallback -->
+    <link
+        rel="stylesheet"
+        href="https://cdn.jsdelivr.net/npm/flag-icons@7.2.3/css/flag-icons.min.css"
+        onerror="this.onerror=null; this.href='{{ url_for('static', path='shared/css/vendor/flag-icons.min.css') }}';"
+    />
 
     <!-- Alpine Cloak -->
     <style>