feat: 模型级联菜单、逐回合分叉、折叠态会话切换、划选引用与界面缩放

- 模型菜单改为级联悬浮结构：服务商独立成行，悬浮其上时模型列表浮出左侧；

apps/desktop/src/preload/index.ts

@@ -1,4 +1,4 @@
-import { contextBridge, ipcRenderer } from 'electron'
+import { contextBridge, ipcRenderer, webFrame } from 'electron'
 import type {
   AgentUiEvent,
   AppSettings,
@@ -28,6 +28,15 @@ const api = {
   minimize: (): Promise<void> => ipcRenderer.invoke('win:minimize'),
   toggleMaximize: (): Promise<void> => ipcRenderer.invoke('win:toggleMaximize'),
   close: (): Promise<void> => ipcRenderer.invoke('win:close'),
+
+  // 界面缩放（Ctrl+= / Ctrl+- / Ctrl+0），factor 为相对倍率，限制在 50%–200%
+  zoom: (factor: number): void => {
+    const next = Math.min(2, Math.max(0.5, webFrame.getZoomFactor() * factor))
+    webFrame.setZoomFactor(next)
+  },
+  zoomReset: (): void => {
+    webFrame.setZoomFactor(1)
+  },
   onWindowState: (callback: (maximized: boolean) => void): (() => void) => {
     const listener = (_e: Electron.IpcRendererEvent, maximized: boolean): void => callback(maximized)
     ipcRenderer.on('win:state', listener)
@@ -78,6 +87,8 @@ const api = {
   navigateTree: (entryId: string, options?: NavigateTreeOptions): Promise<void> =>
     ipcRenderer.invoke('session:navigate', entryId, options),
   forkSession: (): Promise<void> => ipcRenderer.invoke('session:fork'),
+  forkSessionAt: (entryId: string, position: 'at' | 'before'): Promise<void> =>
+    ipcRenderer.invoke('session:forkAt', entryId, position),
   getSettings: (): Promise<AppSettings> => ipcRenderer.invoke('settings:get'),
   getSystemPrompt: (): Promise<string> => ipcRenderer.invoke('settings:getSystemPrompt'),
   setThemeColor: (id: string): Promise<void> => ipcRenderer.invoke('settings:setTheme', id),