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/main/goodbuddy-config-service.test.ts

@@ -103,12 +103,13 @@ describe('GoodBuddyConfigService', () => {
     const current = await application.get()
     const applicationNavigation = { ...current.applicationNavigation, order: [...current.applicationNavigation.order].reverse() }
     const plan = await service.plan('settings-sync', workspace, {
-      operations: [{ operation: 'application.update', updates: { applicationNavigation } }],
+      operations: [{ operation: 'application.update', updates: { applicationNavigation, magicNoteCanvasPageCount: 8 } }],
     })
     expect(changed).not.toHaveBeenCalled()
     await service.apply('settings-sync', { planId: plan.planId }, new AbortController().signal, async () => true)
     expect(changed).toHaveBeenCalledExactlyOnceWith(await application.get())
     expect(changed.mock.calls[0]![0].applicationNavigation).toEqual(applicationNavigation)
+    expect((await service.getSnapshot()).application.magicNoteCanvasPageCount).toBe(8)
     unsubscribe()
     await application.update({ localInferenceEnabled: false })
     expect(changed).toHaveBeenCalledOnce()