feat: add shared templates and static assets infrastructure

Add shared template infrastructure and static assets:
- Shared Jinja2 templates for reusable components
- Favicon for branding
- Local Tailwind CSS fallback
- Shop CSS styles directory

This provides the foundation for consistent UI components across
admin, vendor, and shop frontends with CDN fallback support.

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

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

app/templates/shared/cdn-fallback.html

@@ -0,0 +1,28 @@
+{# app/templates/shared/cdn-fallback.html #}
+{# CDN with Local Fallback Pattern #}
+{# This partial handles loading CDN resources with automatic fallback to local copies #}
+
+{# Tailwind CSS with fallback #}
+<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/tailwindcss@2.2.19/dist/tailwind.min.css"
+      onerror="this.onerror=null; this.href='{{ url_for('static', path='shared/css/tailwind.min.css') }}';">
+
+{# Alpine.js with fallback - must be loaded at the end of body #}
+{# Usage: Include this partial at the bottom of your template, before page-specific scripts #}
+<script>
+// Alpine.js CDN with fallback
+(function() {
+    var script = document.createElement('script');
+    script.defer = true;
+    script.src = 'https://cdn.jsdelivr.net/npm/alpinejs@3.13.3/dist/cdn.min.js';
+
+    script.onerror = function() {
+        console.warn('Alpine.js CDN failed, loading local copy...');
+        var fallbackScript = document.createElement('script');
+        fallbackScript.defer = true;
+        fallbackScript.src = '{{ url_for("static", path="shared/js/vendor/alpine.min.js") }}';
+        document.head.appendChild(fallbackScript);
+    };
+
+    document.head.appendChild(script);
+})();
+</script>