Panic in WebSocket client: nil *websocket.Conn due to setting c.conn to nil

### Describe the Bug

We’re seeing a runtime panic coming from the WebSocket client when the connection is being reused / reconnected:

```text
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0xc0 pc=0xc1e197]

goroutine 1289578 [running]:
github.com/gorilla/websocket.(*Conn).NextReader(0x0)
/go/pkg/mod/github.com/gorilla/websocket@v1.5.3/conn.go:1000 +0x17
github.com/gorilla/websocket.(*Conn).ReadJSON(0xf77860?, {0xe9d720, 0xc002bb5d40})
/go/pkg/mod/github.com/gorilla/websocket@v1.5.3/json.go:50 +0x25
github.com/fosrl/newt/websocket.(*Client).readPumpWithDisconnectDetection(...)
/app/websocket/client.go:681 +0x15c
```

Note the receiver for `(*websocket.Conn).NextReader` is `0x0`, so the panic occurs because we’re calling `ReadJSON` on a `nil` `*websocket.Conn`.

**Relevant code paths**

* `readPumpWithDisconnectDetection` calls:

```go
err := c.conn.ReadJSON(&msg)
```

* `pingMonitor` calls:

```go
if c.conn == nil {
return
}
c.writeMux.Lock()
err := c.conn.WriteControl(...)
c.writeMux.Unlock()
```

* `reconnect` (and related shutdown logic) closes the current connection **and sets `c.conn = nil`** before/while these goroutines are still running.

This means there’s a race where `reconnect` sets `c.conn` to `nil` between loop iterations in `readPumpWithDisconnectDetection` or `pingMonitor`. On the next iteration those goroutines call methods on `c.conn` and we end up in `(*Conn).NextReader(0x0)` → nil pointer dereference.

**Proposed direction**

For now, the minimal fix is simply **not to assign `c.conn = nil` while reader/writer goroutines are still active**. Closing the underlying `*websocket.Conn` is enough to cause `ReadJSON` / `WriteControl` to return an error, which we already handle with reconnect logic. The extra `nil` assignment is what turns a clean error into a panic.

**or**

If you wish to keep the current setting to `nil` then before calling `READJSON` we must check if the connection is `nil` and return but this may cause duplicate re connection attempts.

### Environment

- OS Type & Version: (e.g., Ubuntu 22.04)
- Pangolin Version:
- Gerbil Version:
- Traefik Version:
- Newt Version:
- Olm Version: (if applicable)

### To Reproduce

Simply a race condition and hard to replicate

### Expected Behavior

Panic should never occur during runtime