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/generic.html

@@ -1,31 +1,22 @@
+{# app/templates/shop/errors/generic.html #}
+{# Generic error page - fallback for any error code #}
 {% extends "shop/errors/base.html" %}
 
 {% block icon %}⚠️{% endblock %}
 
 {% block title %}{{ status_code }} - {{ status_name }}{% 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 %}
+{% block action_buttons %}
+<a href="{{ base_url }}shop/"
+   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">
+    Continue Shopping
+</a>
+<a href="javascript:history.back()"
+   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 Back
+</a>
+{% endblock %}
 
-<div class="error-icon">⚠️</div>
-<div class="status-code">{{ status_code }}</div>
-<div class="status-name">{{ status_name }}</div>
-<div class="error-message">{{ message }}</div>
-
-<div class="action-buttons">
-    <a href="{{ base_url }}shop/" class="btn btn-primary">Continue Shopping</a>
-    <a href="javascript:history.back()" class="btn btn-secondary">Go Back</a>
-</div>
-
-<div class="support-link">
-    Need assistance? <a href="{{ base_url }}shop/contact">Contact us</a>
-</div>
-
-{% if vendor %}
-<div class="vendor-info">
-    {{ vendor.name }}
-</div>
-{% endif %}
-{% endblock %}
+{% block support_link %}
+Need assistance? <a href="{{ base_url }}shop/contact" class="text-theme-primary font-semibold hover:underline">Contact us</a>
+{% endblock %}