fix(auth): resolve Keycloak OIDC issuer mismatch and API proxy routing

- Bypass NextAuth OIDC discovery with explicit token/userinfo endpoints using internal Docker DNS, avoiding 'issuer string did not match' errors.
- Fix next.config.ts API route interception that incorrectly forwarded NextAuth routes to backend by using 'fallback' rewrites.
- Add 'Use different credentials' button to login page and AuthGuard for clearing stale sessions.

frontend/src/auth/auth.ts

@@ -19,12 +19,26 @@ declare module "next-auth" {
   }
 }
 
+// In Docker, the Next.js server reaches Keycloak via internal hostname
+// (keycloak:8080) but the browser uses localhost:8080. Explicit endpoint
+// URLs bypass OIDC discovery, avoiding issuer mismatch validation errors.
+const issuerPublic = process.env.KEYCLOAK_ISSUER!
+const issuerInternal = process.env.KEYCLOAK_ISSUER_INTERNAL || issuerPublic
+const oidcPublic = `${issuerPublic}/protocol/openid-connect`
+const oidcInternal = `${issuerInternal}/protocol/openid-connect`
+
 export const { handlers, signIn, signOut, auth } = NextAuth({
   providers: [
     KeycloakProvider({
       clientId: process.env.KEYCLOAK_CLIENT_ID!,
       clientSecret: process.env.KEYCLOAK_CLIENT_SECRET!,
-      issuer: process.env.KEYCLOAK_ISSUER!,
+      issuer: issuerPublic,
+      authorization: {
+        url: `${oidcPublic}/auth`,
+        params: { scope: "openid email profile" },
+      },
+      token: `${oidcInternal}/token`,
+      userinfo: `${oidcInternal}/userinfo`,
     })
   ],
   callbacks: {