fix(auth): resolve broken signup/login flow end-to-end

Three root causes fixed:

1. CSRF blocked all backend POSTs — Spring Security's CSRF filter ran
   before permitAll() authorization, returning 401 for signup and login.
   Disabled CSRF since SvelteKit is the only client (never the browser
   directly) and handles its own CSRF via Origin header checks.

2. Login/signup didn't establish Spring Security authentication — they
   stored email in the HTTP session manually but never set the
   SecurityContext, so Principal in /v1/auth/me was always null and
   hooks.server.ts redirected every authenticated request to /login.
   Fixed with authenticateInSession() helper that sets and persists
   the SecurityContext under SPRING_SECURITY_CONTEXT_KEY. Login also
   now invalidates the old session before creating a new one to prevent
   session fixation.

3. redirect() missing throw in hooks.server.ts, signup action, and
   login action — SvelteKit never saw the redirect, so pages silently
   reloaded with no navigation. Also forward JSESSIONID from backend
   response to browser explicitly, since SvelteKit does not
   auto-forward Set-Cookie for cross-origin server-side fetches.

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

backend/src/main/java/com/recipeapp/auth/AuthController.java

@@ -7,9 +7,15 @@ import jakarta.servlet.http.HttpSession;
 import jakarta.validation.Valid;
 import org.springframework.http.HttpStatus;
 import org.springframework.http.ResponseEntity;
+import org.springframework.security.authentication.UsernamePasswordAuthenticationToken;
+import org.springframework.security.core.authority.SimpleGrantedAuthority;
+import org.springframework.security.core.context.SecurityContext;
+import org.springframework.security.core.context.SecurityContextHolder;
+import org.springframework.security.web.context.HttpSessionSecurityContextRepository;
 import org.springframework.web.bind.annotation.*;
 
 import java.security.Principal;
+import java.util.List;
 
 @RestController
 @RequestMapping("/v1/auth")
@@ -26,7 +32,7 @@ public class AuthController {
             @Valid @RequestBody SignupRequest request,
             HttpServletRequest httpRequest) {
         UserResponse user = authService.signup(request);
-        httpRequest.getSession(true);
+        authenticateInSession(user.email(), "user", httpRequest);
         return ResponseEntity.status(HttpStatus.CREATED).body(ApiResponse.success(user));
     }
 
@@ -35,11 +41,25 @@ public class AuthController {
             @Valid @RequestBody LoginRequest request,
             HttpServletRequest httpRequest) {
         UserResponse user = authService.login(request);
-        HttpSession session = httpRequest.getSession(true);
-        session.setAttribute("user_email", user.email());
+        // Session fixation protection: invalidate old session before creating new one
+        var oldSession = httpRequest.getSession(false);
+        if (oldSession != null) {
+            oldSession.invalidate();
+        }
+        authenticateInSession(user.email(), user.systemRole() != null ? user.systemRole() : "user", httpRequest);
         return ResponseEntity.ok(ApiResponse.success(user));
     }
 
+    private void authenticateInSession(String email, String role, HttpServletRequest request) {
+        var auth = UsernamePasswordAuthenticationToken.authenticated(
+                email, null, List.of(new SimpleGrantedAuthority("ROLE_" + role.toUpperCase())));
+        SecurityContext context = SecurityContextHolder.createEmptyContext();
+        context.setAuthentication(auth);
+        SecurityContextHolder.setContext(context);
+        request.getSession(true).setAttribute(
+                HttpSessionSecurityContextRepository.SPRING_SECURITY_CONTEXT_KEY, context);
+    }
+
     @PostMapping("/logout")
     public ResponseEntity<Void> logout(HttpServletRequest httpRequest) {
         HttpSession session = httpRequest.getSession(false);

backend/src/main/java/com/recipeapp/auth/SecurityConfig.java

@@ -10,8 +10,6 @@ import org.springframework.security.crypto.bcrypt.BCryptPasswordEncoder;
 import org.springframework.security.crypto.password.PasswordEncoder;
 import org.springframework.security.web.SecurityFilterChain;
 import org.springframework.security.web.authentication.HttpStatusEntryPoint;
-import org.springframework.security.web.csrf.CookieCsrfTokenRepository;
-import org.springframework.security.web.csrf.CsrfTokenRequestAttributeHandler;
 
 @Configuration
 @EnableWebSecurity
@@ -20,9 +18,7 @@ public class SecurityConfig {
     @Bean
     public SecurityFilterChain securityFilterChain(HttpSecurity http) throws Exception {
         http
-                .csrf(csrf -> csrf
-                        .csrfTokenRepository(CookieCsrfTokenRepository.withHttpOnlyFalse())
-                        .csrfTokenRequestHandler(new CsrfTokenRequestAttributeHandler()))
+                .csrf(csrf -> csrf.disable())
                 .authorizeHttpRequests(auth -> auth
                         .requestMatchers("/v1/auth/signup", "/v1/auth/login").permitAll()
                         .requestMatchers("/swagger-ui/**", "/v3/api-docs/**").permitAll()