[proxy] Support WebSocket (#5312)

* Fix WebSocket support by implementing Hijacker interface

Add responsewriter.PassthroughWriter to preserve optional HTTP interfaces
(Hijacker, Flusher, Pusher) when wrapping http.ResponseWriter in middleware.

Without this delegation:
 - WebSocket connections fail (can't hijack the connection)
 - Streaming breaks (can't flush buffers)
 - HTTP/2 push doesn't work

* Add HijackTracker to manage hijacked connections during graceful shutdown

* Refactor HijackTracker to use middleware for tracking hijacked connections

* Refactor server handler chain setup for improved readability and maintainability

proxy/internal/accesslog/middleware.go

@@ -9,6 +9,7 @@ import (
 	"github.com/rs/xid"
 
 	"github.com/netbirdio/netbird/proxy/internal/proxy"
+	"github.com/netbirdio/netbird/proxy/internal/responsewriter"
 	"github.com/netbirdio/netbird/proxy/web"
 )
 
@@ -27,8 +28,8 @@ func (l *Logger) Middleware(next http.Handler) http.Handler {
 
 		// Use a response writer wrapper so we can access the status code later.
 		sw := &statusWriter{
-			w:      w,
-			status: http.StatusOK,
+			PassthroughWriter: responsewriter.New(w),
+			status:            http.StatusOK,
 		}
 
 		// Resolve the source IP using trusted proxy configuration before passing

proxy/internal/accesslog/statuswriter.go

@@ -1,26 +1,18 @@
 package accesslog
 
 import (
-	"net/http"
+	"github.com/netbirdio/netbird/proxy/internal/responsewriter"
 )
 
-// statusWriter is a simple wrapper around an http.ResponseWriter
-// that captures the setting of the status code via the WriteHeader
-// function and stores it so that it can be retrieved later.
+// statusWriter captures the HTTP status code from WriteHeader calls.
+// It embeds responsewriter.PassthroughWriter which handles all the optional
+// interfaces (Hijacker, Flusher, Pusher) automatically.
 type statusWriter struct {
-	w      http.ResponseWriter
+	*responsewriter.PassthroughWriter
 	status int
 }
 
-func (w *statusWriter) Header() http.Header {
-	return w.w.Header()
-}
-
-func (w *statusWriter) Write(data []byte) (int, error) {
-	return w.w.Write(data)
-}
-
 func (w *statusWriter) WriteHeader(status int) {
 	w.status = status
-	w.w.WriteHeader(status)
+	w.PassthroughWriter.WriteHeader(status)
 }