feat: implement customer authentication with JWT tokens

Implement secure customer authentication system with dedicated JWT tokens,
separate from admin/vendor authentication.

Backend Changes:
- Add customer JWT token support in deps.py
  - New get_current_customer_from_cookie_or_header dependency
  - Validates customer-specific tokens with type checking
  - Returns Customer object instead of User for shop routes
- Extend AuthService with customer token support
  - Add verify_password() method
  - Add create_access_token_with_data() for custom token payloads
- Update CustomerService authentication
  - Generate customer-specific JWT tokens with type="customer"
  - Use vendor-scoped customer lookup
- Enhance exception handler
  - Sanitize validation errors to prevent password leaks in logs
  - Fix shop login redirect to support multi-access routing
- Improve vendor context detection from Referer header
  - Consistent "path" detection method for cookie path logic

Schema Changes:
- Rename UserLogin.username to email_or_username for flexibility
- Update field validators accordingly

API Changes:
- Update admin/vendor auth endpoints to use email_or_username
- Customer auth already uses email field correctly

Route Changes:
- Update shop account routes to use Customer dependency
- Add /account redirect (without trailing slash)
- Change parameter names from current_user to current_customer

Frontend Changes:
- Update login forms to use email_or_username in API calls
- Change button text from "Log in" to "Sign in" for consistency
- Improve loading spinner layout with flexbox

Security Improvements:
- Customer tokens scoped to vendor_id
- Token type validation prevents cross-context token usage
- Password inputs redacted from validation error logs

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

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

app/templates/admin/login.html

@@ -69,8 +69,8 @@
 
                             <button type="submit" :disabled="loading"
                                     class="block w-full px-4 py-2 mt-4 text-sm font-medium leading-5 text-center text-white transition-colors duration-150 bg-purple-600 border border-transparent rounded-lg active:bg-purple-600 hover:bg-purple-700 focus:outline-none focus:shadow-outline-purple disabled:opacity-50 disabled:cursor-not-allowed">
-                                <span x-show="!loading">Log in</span>
-                                <span x-show="loading">
+                                <span x-show="!loading">Sign in</span>
+                                <span x-show="loading" class="flex items-center justify-center">
                                     <svg class="inline w-4 h-4 mr-2 animate-spin" 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>