Fix Vite proxy inside Docker and add success pages

- vite.config.ts: proxy target via VITE_API_TARGET env var (falls back to localhost)
- docker-compose.dev.yml: set VITE_API_TARGET=http://backend:8000
- Add /login-success and /register-success placeholder pages
- Show real API error messages in login/register forms

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

changelog/2026-04-12_proxy-fix-success-pages.md

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+# 2026-04-12 — Fix Vite proxy and add success pages
+
+**Timestamp:** 2026-04-12T16:00:00
+
+## Summary
+
+Fixed login/registration failures caused by wrong Vite proxy target inside Docker. Added login and registration success pages. Improved error messages to show actual API responses.
+
+## Root Cause
+
+Vite's dev server proxy was targeting `http://localhost:8000`. Inside the Docker network, `localhost` resolves to the frontend container itself (not the backend). The correct target inside Docker is `http://backend:8000` (Docker service name).
+
+## Files Modified
+
+- `frontend/vite.config.ts` — proxy target now reads from `process.env.VITE_API_TARGET`, falling back to `http://localhost:8000` for local dev without Docker
+- `docker-compose.dev.yml` — added `VITE_API_TARGET: http://backend:8000` to frontend environment
+- `frontend/src/App.tsx` — added `/login-success` and `/register-success` routes
+- `frontend/src/hooks/useAuth.ts` — redirects to `/login-success` after login
+- `frontend/src/pages/LoginPage.tsx` — parses real API error from response instead of generic message
+- `frontend/src/pages/RegisterPage.tsx` — redirects to `/register-success` on success; shows real API validation errors
+
+## Files Added
+
+- `frontend/src/pages/LoginSuccessPage.tsx` — placeholder: "Login successful"
+- `frontend/src/pages/RegisterSuccessPage.tsx` — placeholder: "Registration successful" with link to sign in