fix: storefront login 403, cookie path, double-storefront URLs, and auth redirects

- Extract store/platform context from Referer header for storefront API requests
  (StoreContextMiddleware and PlatformContextMiddleware) so login POST works in
  dev mode where API paths lack /platforms/{code}/ prefix
- Set customer token cookie path to "/" for cross-route compatibility
- Fix double storefront in URLs: replace {{ base_url }}storefront/ with {{ base_url }}
  across all 24 storefront templates
- Fix auth error redirect to include platform prefix and use store_code
- Update seed script to output correct storefront login URLs
- Add 20 new unit tests covering all fixes; fix 9 pre-existing test failures

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

scripts/seed/seed_demo.py

@@ -1382,9 +1382,10 @@ def print_summary(db: Session):
         print(f"  {store.name} ({store.store_code}):")
         if platform_codes:
             for pc in platform_codes:
-                print(f"    [{pc}] Storefront: {base}/platforms/{pc}/storefront/{store.store_code}/")
-                print(f"    [{pc}] Dashboard:  {base}/platforms/{pc}/store/{store.store_code}/")
-                print(f"    [{pc}] Login:      {base}/platforms/{pc}/store/{store.store_code}/login")
+                print(f"    [{pc}] Storefront:      {base}/platforms/{pc}/storefront/{store.store_code}/")
+                print(f"    [{pc}] Customer login:  {base}/platforms/{pc}/storefront/{store.store_code}/account/login")
+                print(f"    [{pc}] Store dashboard: {base}/platforms/{pc}/store/{store.store_code}/")
+                print(f"    [{pc}] Staff login:     {base}/platforms/{pc}/store/{store.store_code}/login")
         else:
             print("    (!) No platform assigned")
     print()
@@ -1397,7 +1398,7 @@ def print_summary(db: Session):
     print(f"  3. Merchant panel: {base}/merchants/login")
     print(f"  4. Store panel:    {base}/platforms/oms/store/WIZATECH/login")
     print(f"  5. Storefront:     {base}/platforms/oms/storefront/WIZATECH/")
-    print(f"  6. Customer login: {base}/platforms/oms/storefront/WIZATECH/account")
+    print(f"  6. Customer login: {base}/platforms/oms/storefront/WIZATECH/account/login")
 
 
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