Add configurable HTTP/SOCKS5 network proxy (#10)

Adds a Settings UI to route Vega through an HTTP or SOCKS5 proxy (e.g. Tor).

Covers both network stacks: the webview (proxy_url applied at build time, so
relay WebSocket traffic is proxied) and all five Rust-side plugin-http call
sites (converted to fetchWithProxy). The auto-updater's check and download are
also routed via check({ proxy }), so update traffic no longer bypasses the
proxy and leaks the user's IP.

socks5h:// is intentionally rejected for now: Tauri's webview proxy API only
documents http and socks5, so accepting socks5h globally would imply a DNS-leak
guarantee the webview can't yet make. Tracking a cross-platform DNS-leak test
as follow-up.

By Anderseta.

src-tauri/Cargo.lock

@@ -5435,6 +5435,7 @@ dependencies = [
  "futures-util",
  "hex",
  "keyring",
+ "reqwest 0.13.2",
  "rusqlite",
  "secp256k1",
  "serde",