Refactor form error handling with type guards, enhance API client configuration, and update implementation plan

- Introduced `isAPIErrorArray` type guard to improve error handling in authentication forms, replacing type assertions for better runtime safety.
- Refactored error handling logic across `RegisterForm`, `LoginForm`, `PasswordResetRequestForm`, and `PasswordResetConfirmForm` for unexpected error fallbacks.
- Updated `next.config.ts` and `.eslintrc.json` to exclude generated API client files from linting and align configuration with latest project structure.
- Added comprehensive documentation on Phase 2 completion in `IMPLEMENTATION_PLAN.md`.

frontend/next.config.ts

@@ -11,6 +11,11 @@ const nextConfig: NextConfig = {
             },
         ];
     },
+    // Exclude generated API client from ESLint
+    eslint: {
+        ignoreDuringBuilds: false,
+        dirs: ['src', 'tests'],
+    },
 };
 
 export default nextConfig;