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>

static/shared/css/flags/4x3/de.svg

@@ -0,0 +1,5 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-de" viewBox="0 0 640 480">
+  <path fill="#fc0" d="M0 320h640v160H0z"/>
+  <path fill="#000001" d="M0 0h640v160H0z"/>
+  <path fill="red" d="M0 160h640v160H0z"/>
+</svg>