feat: configure canvas pages sent for note analysis

Canvas analysis previously sent every page. Add a Magic Notes setting for
1-8 canvas pages, defaulting to 1 for new and historical settings. Select
the first N pages in current canvas order before capture and use the same
text/image range for saved entries, drafts and canvas-source todos.

Keep full 50-page saving/export, expose the same setting through config
tools, and use the shared settings select with an inline help icon.
Include pagination, persistence, production IPC and isolated Electron
regressions, including text-only input and scaled page boundaries.

Validation: focused feature tests and isolated Electron regressions passed;
the final compact UI passed 14 tests. Typecheck and lint were reported
passing by the coordinating agent. No full-suite pass is claimed because
the completed full test command exposed no summary. No live model calls
were made for this commit preparation.

Release note: 魔法笔记新增“发送画布页数”设置，可选 1～8 页，默认 1 页，按当前画布顺序发送前 N 页的文字和图片；标题旁问号可查看说明，保存和导出完整画布不受影响。

src/shared/goodbuddy-config-contracts.test.ts

@@ -62,7 +62,8 @@ describe('GoodBuddy configuration contracts', () => {
           operation: 'application.update',
           updates: {
             magicNotesEnabled: true,
-            magicNoteCommentMode: 'after-save-manual'
+            magicNoteCommentMode: 'after-save-manual',
+            magicNoteCanvasPageCount: 8
           }
         },
         {
@@ -161,7 +162,8 @@ describe('GoodBuddy configuration contracts', () => {
         magicNotesEnabled: true,
         magicNotesShowIncompleteTodoCount: true,
         magicNoteCommentMode: 'immediate',
-        magicNoteCommentFormat: 'combined'
+        magicNoteCommentFormat: 'combined',
+        magicNoteCanvasPageCount: 1
       },
       skills: [],
       mcpServers: [