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/shared/magic-notes-contracts.ts

@@ -438,6 +438,10 @@ export type MagicTodoUpdateResult = {
   note: MagicNoteDetail
 }
 
+export type MagicTodoStatus = {
+  incompleteCount: number
+}
+
 export type MagicTodosSnapshot = {
   todos: MagicTodoItem[]
 }