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/main/ipc.ts

@@ -5682,6 +5682,14 @@ export function registerIpcHandlers(
     }
   )
 
+  registerHandler(
+    ipcChannels.magicTodosStatus,
+    (event) => {
+      assertTrustedSender(event, window)
+      return assistantDatabase.getMagicTodoStatus()
+    }
+  )
+
   registerHandler(
     ipcChannels.magicTodosUpdate,
     (event, input: unknown) => {