fix: sandbox network responsiveness under port-binding apps

Running port-binding applications (Jupyter, http.server, NextJS) inside
sandboxes caused severe PTY sluggishness and proxy navigation errors.

Root cause: the CP sandbox proxy and Connect RPC pool shared a single
HTTP transport. Heavy proxy traffic (Jupyter WebSocket, REST polling)
interfered with PTY RPC streams via HTTP/2 flow control contention.

Transport isolation (main fix):
- Add dedicated proxy transport on CP (NewProxyTransport) with HTTP/2
  disabled, separate from the RPC pool transport
- Add dedicated proxy transport on host agent, replacing
  http.DefaultTransport
- Add dedicated envdclient transport with tuned connection pooling
- Replace http.DefaultClient in file streaming RPCs with per-sandbox
  envd client

Proxy path rewriting (navigation fix):
- Add ModifyResponse to rewrite Location headers with /proxy/{id}/{port}
  prefix, handling both root-relative and absolute-URL redirects
- Strip prefix back out in CP subdomain proxy for correct browser
  behavior
- Replace path.Join with string concat in CP Director to preserve
  trailing slashes (prevents redirect loops on directory listings)

Proxy resilience:
- Add dial retry with linear backoff (3 attempts) to handle socat
  startup delay when ports are first detected
- Cache ReverseProxy instances per sandbox+port+host in sync.Map
- Add EvictProxy callback wired into sandbox Manager.Destroy

Buffer and server hardening:
- Increase PTY and exec stream channel buffers from 16 to 256
- Add ReadHeaderTimeout (10s) and IdleTimeout (620s) to host agent
  HTTP server

Network tuning:
- Set TAP device TxQueueLen to 5000 (up from default 1000)
- Add Firecracker tx_rate_limiter (200 MB/s sustained, 100 MB burst)
  to prevent guest traffic from saturating the TAP

internal/envdclient/dialer.go

@@ -2,7 +2,9 @@ package envdclient
 
 import (
 	"fmt"
+	"net"
 	"net/http"
+	"time"
 )
 
 // envdPort is the default port envd listens on inside the guest.
@@ -13,9 +15,19 @@ func baseURL(hostIP string) string {
 	return fmt.Sprintf("http://%s:%d", hostIP, envdPort)
 }
 
-// newHTTPClient returns an http.Client suitable for talking to envd.
-// No special transport is needed — envd is reachable via the host IP
-// through the veth/TAP network path.
+// newHTTPClient returns an http.Client with a dedicated transport for talking
+// to envd. The transport is intentionally separate from http.DefaultTransport
+// so that proxy traffic to user services inside the sandbox cannot interfere
+// with envd RPC connections (PTY streams, exec, file ops).
 func newHTTPClient() *http.Client {
-	return &http.Client{}
+	return &http.Client{
+		Transport: &http.Transport{
+			MaxIdleConnsPerHost: 10,
+			IdleConnTimeout:     90 * time.Second,
+			DialContext: (&net.Dialer{
+				Timeout:   10 * time.Second,
+				KeepAlive: 30 * time.Second,
+			}).DialContext,
+		},
+	}
 }