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/hostagent/server.go

@@ -459,7 +459,7 @@ func (s *Server) WriteFileStream(
 	}
 	httpReq.Header.Set("Content-Type", mpWriter.FormDataContentType())
 
-	resp, err := http.DefaultClient.Do(httpReq)
+	resp, err := client.HTTPClient().Do(httpReq)
 	if err != nil {
 		pw.CloseWithError(err)
 		<-errCh
@@ -504,7 +504,7 @@ func (s *Server) ReadFileStream(
 		return connect.NewError(connect.CodeInternal, fmt.Errorf("create request: %w", err))
 	}
 
-	resp, err := http.DefaultClient.Do(httpReq)
+	resp, err := client.HTTPClient().Do(httpReq)
 	if err != nil {
 		return connect.NewError(connect.CodeInternal, fmt.Errorf("read file stream: %w", err))
 	}