feat: show incomplete Magic Notes todo counts

Magic Notes can now show a default-on incomplete todo badge in the primary
navigation. The badge hides at zero, caps its visual label at 99+ while
retaining the exact accessible count, and refreshes after user or Agent/MCP
note changes.

Platform Features > Magic Notes includes a persistent switch that hides only
the count without changing notes or the Magic Notes entry. Existing settings
migrate with the count enabled.

Release note: 左侧“魔法笔记”入口现在可显示未完成待办数量；可在“平台功能 → 魔法笔记”中关闭，超过 99 项显示“99+”。

src/preload/index.ts

@@ -107,6 +107,7 @@ import type {
   MagicNoteDetail,
   MagicNotesSnapshot,
   MagicTodoItem,
+  MagicTodoStatus,
   MagicTodoUpdateResult,
   MagicTodosSnapshot
 } from '../shared/magic-notes-contracts'
@@ -1110,6 +1111,10 @@ const desktopApi: DesktopApi = {
       ipcRenderer.invoke(
         ipcChannels.magicTodosList
       ) as Promise<MagicTodosSnapshot>,
+    getTodoStatus: () =>
+      ipcRenderer.invoke(
+        ipcChannels.magicTodosStatus
+      ) as Promise<MagicTodoStatus>,
     updateTodo: (input) =>
       ipcRenderer.invoke(
         ipcChannels.magicTodosUpdate,
@@ -1131,6 +1136,15 @@ const desktopApi: DesktopApi = {
           ipcChannels.magicNotesAnalysisEvent,
           handler
         )
+    },
+    onTodoStatusChanged: (listener) => {
+      const handler = (): void => listener()
+      ipcRenderer.on(ipcChannels.magicTodosStatusChanged, handler)
+      return () =>
+        ipcRenderer.removeListener(
+          ipcChannels.magicTodosStatusChanged,
+          handler
+        )
     }
   },
   knowledge: {