feat(tunnel): switch to yamux stream proxying and connection pooling

- Introduce pooled tunnel sessions (TunnelID/DataConnect) on client/server
- Proxy HTTP/HTTPS via raw HTTP over yamux streams; pipe TCP streams directly
- Move UI/stats into internal/shared; refactor CLI tunnel helpers; drop msgpack/hpack legacy

internal/shared/httputil/helpers.go

@@ -0,0 +1,71 @@
+package httputil
+
+import (
+	"fmt"
+	"io"
+	"net/http"
+	"strings"
+)
+
+// CopyHeaders copies all headers from src to dst.
+func CopyHeaders(dst, src http.Header) {
+	for k, vv := range src {
+		for _, v := range vv {
+			dst.Add(k, v)
+		}
+	}
+}
+
+// CleanHopByHopHeaders removes hop-by-hop headers that should not be forwarded.
+func CleanHopByHopHeaders(headers http.Header) {
+	if headers == nil {
+		return
+	}
+
+	if connectionHeaders := headers.Get("Connection"); connectionHeaders != "" {
+		for _, token := range strings.Split(connectionHeaders, ",") {
+			if t := strings.TrimSpace(token); t != "" {
+				headers.Del(http.CanonicalHeaderKey(t))
+			}
+		}
+	}
+
+	for _, key := range []string{
+		"Connection",
+		"Keep-Alive",
+		"Proxy-Authenticate",
+		"Proxy-Authorization",
+		"Te",
+		"Trailer",
+		"Transfer-Encoding",
+		"Proxy-Connection",
+	} {
+		headers.Del(key)
+	}
+}
+
+// WriteProxyError writes an HTTP error response to the writer.
+func WriteProxyError(w io.Writer, code int, msg string) {
+	body := msg
+	resp := &http.Response{
+		StatusCode:    code,
+		Status:        fmt.Sprintf("%d %s", code, http.StatusText(code)),
+		Proto:         "HTTP/1.1",
+		ProtoMajor:    1,
+		ProtoMinor:    1,
+		Header:        make(http.Header),
+		Body:          io.NopCloser(strings.NewReader(body)),
+		ContentLength: int64(len(body)),
+		Close:         true,
+	}
+	resp.Header.Set("Content-Type", "text/plain; charset=utf-8")
+	resp.Header.Set("Content-Length", fmt.Sprintf("%d", len(body)))
+	_ = resp.Write(w)
+	_ = resp.Body.Close()
+}
+
+// IsWebSocketUpgrade checks if the request is a WebSocket upgrade request.
+func IsWebSocketUpgrade(req *http.Request) bool {
+	return strings.EqualFold(req.Header.Get("Upgrade"), "websocket") &&
+		strings.Contains(strings.ToLower(req.Header.Get("Connection")), "upgrade")
+}