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/contracts.ts

@@ -63,6 +63,7 @@ import type {
   MagicNotesSnapshot,
   MagicNoteUpdateInput,
   MagicTodoItem,
+  MagicTodoStatus,
   MagicTodoUpdateInput,
   MagicTodoUpdateResult,
   MagicTodosSnapshot
@@ -1703,6 +1704,7 @@ export type DesktopApi = {
       options: MagicNoteAnalysisOptions
     ) => Promise<MagicNoteDraftAnalysis>
     listTodos: () => Promise<MagicTodosSnapshot>
+    getTodoStatus: () => Promise<MagicTodoStatus>
     updateTodo: (
       input: MagicTodoUpdateInput
     ) => Promise<MagicTodoUpdateResult>
@@ -1713,6 +1715,7 @@ export type DesktopApi = {
     onAnalysisEvent: (
       listener: (event: MagicNoteAnalysisStreamEvent) => void
     ) => () => void
+    onTodoStatusChanged: (listener: () => void) => () => void
   }
   knowledge: {
     getSnapshot: (libraryId?: string) => Promise<KnowledgeSnapshot>