fix: correct base_url calculation in error renderer for path-based routing

Fix error page links not respecting vendor context in path-based routing
by aligning with the pattern used throughout the codebase.

Problem:
- Error pages showed links like /shop/ instead of /vendors/wizamart/shop/
- error_renderer.py was checking request.state.access_method (never set)
- This meant access_method was always None, so base_url defaulted to "/"
- Links ignored vendor context and broke multi-access routing

Root Cause:
- Inconsistent pattern: error_renderer.py used wrong attribute
- Rest of codebase (6 locations) correctly uses vendor_context.get('detection_method')
- No code ever sets request.state.access_method anywhere

Solution:
- Change from: access_method = getattr(request.state, "access_method", None)
- Change to: access_method = vendor_context.get('detection_method', 'unknown')
- Aligns with pattern used in:
  * app/exceptions/handler.py:384 (login redirect)
  * main.py:336 (root redirect)
  * app/routes/shop_pages.py:85, 391 (shop pages)
  * app/api/v1/shop/auth.py:159, 223 (auth endpoints)

Changes:
- Line 263-264: Get vendor_context first, then extract detection_method from it
- Line 266: Now correctly detects path-based access method
- base_url now properly set to /vendors/wizamart/ for path-based routing

Impact:
- ✅ Path-based routing: /vendors/wizamart/shop123 → links to /vendors/wizamart/shop/
- ✅ Direct shop access: /shop/test → links to /shop/ (unchanged)
- ✅ No breaking changes (access_method was never used before)
- ✅ Consistent with entire codebase pattern

Testing:
- curl http://localhost:8000/vendors/wizamart/shop123
  Before: href="/shop/" (wrong)
  After: href="/vendors/wizamart/shop/" (correct)

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

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

app/exceptions/error_renderer.py

@@ -260,8 +260,8 @@ class ErrorPageRenderer:
                     }
 
             # Calculate base_url for shop links
-            access_method = getattr(request.state, "access_method", None)
             vendor_context = getattr(request.state, "vendor_context", None)
+            access_method = vendor_context.get('detection_method', 'unknown') if vendor_context else 'unknown'
             base_url = "/"
             if access_method == "path" and vendor:
                 # Use the full_prefix from vendor_context to determine which pattern was used