Add SSH keepalive, fix SOCKS5 format bug, implement web GUI

[Copilot]

Git SSH operations (e.g. git remote show origin) fail with "Connection closed by port 22" because HTTP proxy doesn't intercept SSH traffic, and idle SSH tunnels get dropped by firewalls. Adds keepalive to fix tunnel stability and a web GUI to surface SOCKS5 SSH proxy configuration.

Changes

internal/proxy/ssh.go — SSH keepalive

• Added keepAlive() goroutine sending keepalive@openssh.com every 30s via ssh.Client.SendRequest
• On timeout (15s), closes the connection and exits cleanly — no goroutine accumulation
• stopChan chan struct{} added to SSHClient for coordinated shutdown; Close() signals the goroutine before tearing down connections

internal/proxy/socks5.go — Format verb typo

• Fixed %S → %s in Warnf call ([%SOCKS5] was emitting a literal bad verb)

gui/app.go + cmd/gotun-gui/main.go — Web-based control panel

Implements the planned "Tray/GUI frontend" roadmap item. No external dependencies — pure stdlib HTTP server serving a single-page UI.

• REST API: GET /api/status, POST /api/start, POST /api/stop
• When SOCKS5 is active, the UI surfaces the ~/.ssh/config snippet required to route git SSH through the proxy:

Host github.com
    ProxyCommand nc -x :1080 %h %p
    ServerAliveInterval 30
    ServerAliveCountMax 3

Alternative commands (ncat, connect-proxy) are also shown for environments where nc lacks SOCKS5 (-x) support.

Makefile

• build target now produces both gotun and gotun-gui binaries

Original prompt

解释：❯ git remote show origin 🐍 base  11:49:51
Connection closed by 20.205.243.166 port 22
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.

The user has attached the following file paths as relevant context:

• gui/app.go

创建自 VS Code。

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