fix: replace inline SVGs with $icon() helper (FE-002)

Convert 90 inline SVG icons to use the shared $icon() helper function
across shop and vendor templates for consistency and maintainability.

Templates updated:
- Shop: checkout, products, login, register, forgot/reset-password
- Shop account: addresses, dashboard, messages, order-detail, orders, profile
- Vendor: billing, login, onboarding, team, landing pages (4 variants)

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

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

app/templates/vendor/billing.html

@@ -382,10 +382,7 @@
                                 class="px-4 py-2 text-sm font-medium rounded-lg transition-colors">
                             <template x-if="purchasingAddon === addon.code">
                                 <span class="flex items-center">
-                                    <svg class="animate-spin -ml-1 mr-2 h-4 w-4" xmlns="http://www.w3.org/2000/svg" fill="none" viewBox="0 0 24 24">
-                                        <circle class="opacity-25" cx="12" cy="12" r="10" stroke="currentColor" stroke-width="4"></circle>
-                                        <path class="opacity-75" fill="currentColor" d="M4 12a8 8 0 018-8V0C5.373 0 0 5.373 0 12h4zm2 5.291A7.962 7.962 0 014 12H0c0 3.042 1.135 5.824 3 7.938l3-2.647z"></path>
-                                    </svg>
+                                    <span class="-ml-1 mr-2 h-4 w-4" x-html="$icon('spinner', 'h-4 w-4 animate-spin')"></span>
                                     Processing...
                                 </span>
                             </template>