Port remotesrv and HTTP remote client to Windows (Winsock + Win32 threads)

## Summary

PR #200 added Windows build support but stubbed out the networking code:

- **doltlite_http_remote.c**: `doltliteRegisterHttpRemote()` is a no-op on Windows
- **doltlite_remotesrv.c**: `doltliteServerCreate()` returns NULL with error on Windows

This means `dolt_remote_add` / push / fetch / clone and the standalone `doltlite-remotesrv` binary don't work on Windows.

## What needs to happen

1. Replace BSD socket calls with Winsock2 (`WSAStartup`, `socket`, `connect`, `send`, `recv`, `closesocket`)
2. Replace `pthread_create`/`pthread_join` with `CreateThread`/`WaitForSingleObject`
3. Replace `poll()` with `WSAPoll()` (Vista+) or `select()`
4. Add `#ifdef _WIN32` / `#else` branches or use a thin abstraction layer
5. Link with `-lws2_32` on Windows

## Files

- `src/doltlite_http_remote.c` — HTTP client (sockets only, no threads)
- `src/doltlite_remotesrv.c` — HTTP server (sockets + threads + poll)