fix: customer authentication and shop error page styling

## Customer Authentication Fixes
- Fix get_current_customer_api to properly decode customer tokens (was using User model)
- Add _validate_customer_token() helper for shared customer token validation
- Add vendor validation: token.vendor_id must match request URL vendor
- Block admin/vendor tokens from shop endpoints (type != "customer")
- Update get_current_customer_optional to use proper customer token validation
- Customer auth functions now return Customer object (not User)

## Shop Orders API
- Update orders.py to receive Customer directly from auth dependency
- Remove broken get_customer_from_user() helper
- Use VendorNotFoundException instead of HTTPException

## Shop Error Pages
- Fix all error templates (400, 401, 403, 404, 422, 429, 500, 502, generic)
- Templates were using undefined CSS classes (.btn, .status-code, etc.)
- Now properly extend base.html and override specific blocks
- Use Tailwind utility classes for consistent styling

## Documentation
- Update docs/api/authentication.md with new Customer return types
- Document vendor validation security features
- Update docs/api/authentication-quick-reference.md examples

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

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

app/templates/shop/errors/422.html

@@ -1,46 +1,35 @@
+{# app/templates/shop/errors/422.html #}
+{# 422 Unprocessable Entity error page - validation errors #}
 {% extends "shop/errors/base.html" %}
 
 {% block icon %}📝{% endblock %}
 
 {% block title %}422 - Invalid Information{% endblock %}
 
-{% block content %}
-{% if vendor and theme and theme.branding and theme.branding.logo %}
-<img src="{{ theme.branding.logo }}" alt="{{ vendor.name }}" class="vendor-logo">
-{% endif %}
-
-<div class="error-icon">📝</div>
-<div class="status-code">422</div>
-<div class="status-name">Please Check Your Information</div>
-<div class="error-message">
-    Some of the information you provided isn't valid. Please review the form and try again.
-</div>
-
+{% block extra_content %}
 {% if details and details.validation_errors %}
-<div style="margin: 2rem auto; max-width: 400px; text-align: left; background: #fef2f2; padding: 1.5rem; border-radius: 0.75rem; border-left: 4px solid var(--color-primary);">
-    <h3 style="color: var(--color-text); font-size: 0.875rem; margin-bottom: 0.75rem; font-weight: 600;">Please correct:</h3>
-    <ul style="list-style: none; padding: 0; margin: 0;">
+<div class="my-8 mx-auto max-w-md text-left bg-red-50 p-6 rounded-xl border-l-4 border-theme-primary">
+    <h3 class="text-gray-700 text-sm mb-3 font-semibold">Please correct:</h3>
+    <ul class="list-none p-0 m-0">
     {% for error in details.validation_errors %}
-        <li style="margin-bottom: 0.5rem; color: #7f1d1d; font-size: 0.875rem;">
-            • {{ error.msg }}
-        </li>
+        <li class="mb-2 text-red-800 text-sm">• {{ error.msg }}</li>
     {% endfor %}
     </ul>
 </div>
 {% endif %}
+{% endblock %}
 
-<div class="action-buttons">
-    <a href="javascript:history.back()" class="btn btn-primary">Go Back and Fix</a>
-    <a href="{{ base_url }}shop/" class="btn btn-secondary">Go to Home</a>
-</div>
+{% block action_buttons %}
+<a href="javascript:history.back()"
+   class="inline-flex items-center px-8 py-4 rounded-xl font-semibold text-white bg-theme-primary hover:opacity-90 hover:-translate-y-0.5 transition-all shadow-lg">
+    Go Back and Fix
+</a>
+<a href="{{ base_url }}shop/"
+   class="inline-flex items-center px-8 py-4 rounded-xl font-semibold text-theme-primary border-2 border-theme-primary hover:bg-theme-primary hover:text-white hover:-translate-y-0.5 transition-all">
+    Go to Home
+</a>
+{% endblock %}
 
-<div class="support-link">
-    Having trouble? <a href="{{ base_url }}shop/contact">We're here to help</a>
-</div>
-
-{% if vendor %}
-<div class="vendor-info">
-    {{ vendor.name }}
-</div>
-{% endif %}
-{% endblock %}
+{% block support_link %}
+Having trouble? <a href="{{ base_url }}shop/contact" class="text-theme-primary font-semibold hover:underline">We're here to help</a>
+{% endblock %}