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/1x1/gb.svg

@@ -0,0 +1,7 @@
+<svg xmlns="http://www.w3.org/2000/svg" id="flag-icons-gb" viewBox="0 0 512 512">
+  <path fill="#012169" d="M0 0h512v512H0z"/>
+  <path fill="#FFF" d="M512 0v64L322 256l190 187v69h-67L254 324 68 512H0v-68l186-187L0 74V0h62l192 188L440 0z"/>
+  <path fill="#C8102E" d="m184 324 11 34L42 512H0v-3zm124-12 54 8 150 147v45zM512 0 320 196l-4-44L466 0zM0 1l193 189-59-8L0 49z"/>
+  <path fill="#FFF" d="M176 0v512h160V0zM0 176v160h512V176z"/>
+  <path fill="#C8102E" d="M0 208v96h512v-96zM208 0v512h96V0z"/>
+</svg>