feat: add external proxy support (HTTP/SOCKS5 dialer)

[pranavthakur0-0]

Here's the PR description as plain text:

What does this PR do?

This PR adds external proxy support to the Zen proxy, allowing all outbound connections to be routed through an HTTP or SOCKS5 upstream proxy. This enables Zen to work alongside VPNs, corporate proxies, and tools like Tor that use proxy tunneling.

How it works

A new dialer field (proxy.Dialer) is added to the Proxy struct, defaulting to proxy.Direct (direct TCP connection — identical to current behavior). When an external proxy is configured via WithExternalProxy(), the dialer is swapped to either a proxy.SOCKS5 dialer (from golang.org/x/net/proxy, an existing dependency) or a custom httpConnectDialer for HTTP CONNECT proxies with optional Basic auth. All outbound connection points (requestTransport, tunnel(), proxyWebsocket(), proxyWebsocketTLS()) now use p.dialer.Dial instead of net.Dial / p.netDialer.Dial. Errors during SOCKS5 dialer creation and unsupported protocol values are logged rather than silently ignored.

Files changed

proxy/upstream.go (NEW) — ExternalProxyConfig struct, ProxyOption type, WithExternalProxy() functional option, httpConnectDialer with Basic auth support

proxy/proxy.go — Added dialer proxy.Dialer field; NewProxy() accepts ...ProxyOption; requestTransport.Dial and tunnel() use p.dialer.Dial

proxy/websocket.go — proxyWebsocket() and proxyWebsocketTLS() use p.dialer.Dial with manual TLS upgrade

How did you verify your code works?

go build ./... passes with no errors. No regression: default behavior is unchanged since proxy.Direct is the default dialer.

What are the relevant issues?

Closes ZenPrivacy/zen-desktop#609

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📥 Commits

Reviewing files that changed from the base of the PR and between 8488c49 and 7011064.

📒 Files selected for processing (3)

• proxy/proxy.go
• proxy/upstream.go
• proxy/websocket.go

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