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/preload.js

@@ -0,0 +1,27 @@
+const { contextBridge, ipcRenderer } = require("electron");
+
+contextBridge.exposeInMainWorld("modelrouter", {
+  win: {
+    minimize: () => ipcRenderer.invoke("window:minimize"),
+    toggleMaximize: () => ipcRenderer.invoke("window:toggleMaximize"),
+    close: () => ipcRenderer.invoke("window:close"),
+    isMaximized: () => ipcRenderer.invoke("window:isMaximized"),
+  },
+  core: {
+    state: () => ipcRenderer.invoke("core:state"),
+    start: () => ipcRenderer.invoke("core:start"),
+    stop: () => ipcRenderer.invoke("core:stop"),
+    restart: () => ipcRenderer.invoke("core:restart"),
+    probe: () => ipcRenderer.invoke("core:probe"),
+    key: () => ipcRenderer.invoke("core:key"),
+    onState: (cb) => ipcRenderer.on("core:state", (_e, d) => cb(d)),
+  },
+  settings: {
+    get: () => ipcRenderer.invoke("settings:get"),
+    set: (patch) => ipcRenderer.invoke("settings:set", patch),
+    logFile: () => ipcRenderer.invoke("settings:logFile"),
+  },
+  shell: {
+    openPath: (p) => ipcRenderer.invoke("shell:openPath", p),
+  },
+});