feat(gui): Electron desktop with embedded core, tray, autostart, win/linux packaging

- cmd/gui: Electron shell (Clash-Verge style) embedding the full WebUI 1:1
  - embedded llmsproxy core (luajit) with auto-generated profile
  - key stored in keys[] (non-seed) so no replace-the-key warning
  - gw_key cookie injection: web UI works without login
  - side-rail toggles for autostart / silent start
  - system tray with status + controls, silent start (--silent)
  - win cross-build (mingw luajit exe + dll) / deb / AppImage via electron-builder
- Makefile: build / gui / gui-dist / gui-deb / gui-win targets
- README: desktop GUI section
- lua(adapter): opencode normalizes non-whitelisted roles to system

cmd/gui/icon.svg

@@ -0,0 +1,11 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 34 34" width="34" height="34">
+  <defs>
+    <linearGradient id="g" x1="0" y1="0" x2="1" y2="1">
+      <stop offset="0" stop-color="#3f6ef5"/>
+      <stop offset="1" stop-color="#6a8ffb"/>
+    </linearGradient>
+  </defs>
+  <rect width="34" height="34" rx="10" fill="url(#g)"/>
+  <text x="17" y="24" text-anchor="middle" font-family="Arial,sans-serif" font-size="17" font-weight="700" fill="#ffffff">M</text>
+</svg>