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/tauri.conf.json

@@ -10,15 +10,7 @@
     "frontendDist": "../dist"
   },
   "app": {
-    "windows": [
-      {
-        "title": "Vega",
-        "width": 1200,
-        "height": 800,
-        "minWidth": 900,
-        "minHeight": 600
-      }
-    ],
+    "windows": [],
     "security": {
       "csp": null
     }