refactor: convert auth register to Server Action

- Add src/app/actions/auth.ts with registerUser
- Update RegisterForm to use Server Action
- Remove api/auth/register route

docs/server-actions-plan.md

@@ -14,6 +14,7 @@
 | `POST /api/komga/libraries/[libraryId]/scan` | `scanLibrary()` | ✅ Done |
 | `POST /api/komga/config` | `saveKomgaConfig()` | ✅ Done |
 | `PUT /api/user/password` | `changePassword()` | ✅ Done |
+| `POST /api/auth/register` | `registerUser()` | ✅ Done |
 
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