fix(spa): add /register and /onboarding to SPA forward routes

These routes were missing from SpaController, so requests to
/platform/register and /platform/onboarding had no handler. Spring
forwarded to /error, which isn't in the permitAll() list, resulting
in a 401 instead of serving the SPA.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/main/java/net/siegeln/cameleer/saas/config/SpaController.java

@@ -7,7 +7,7 @@ import org.springframework.web.bind.annotation.RequestMapping;
 public class SpaController {
 
     @RequestMapping(value = {
-        "/", "/login", "/callback",
+        "/", "/login", "/register", "/callback", "/onboarding",
         "/vendor/**", "/tenant/**"
     })
     public String forward() {