Add a single global-proxy switch that routes the backend's outbound traffic
through a configured HTTP/HTTPS/SOCKS proxy, for deployments that cannot reach
overseas APIs directly or must use a unified egress.
- ProxyManager installs the proxy via a default ProxySelector (honored by
java.net.http and HttpURLConnection), the proxy system properties, and a
--proxy-server arg for the browser tool; restores direct egress when
disabled. One switch covers LLM, web search, media generation, channels,
MCP and the browser.
- SOCKS applies to the HttpURLConnection-based egress only; the java.net.http
LLM/streaming path uses an HTTP proxy, and the UI states this.
- New Settings -> Network Proxy page: enable toggle, address, bypass list,
test-connection, and a coverage summary. Config stored as key/value in
mate_system_setting (no migration).
refs #109
