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/application-settings-store.ts

@@ -258,7 +258,8 @@ export const defaultApplicationSettings: ApplicationSettings = {
   magicNotesEnabled: true,
   magicNotesShowIncompleteTodoCount: true,
   magicNoteCommentMode: 'immediate',
-  magicNoteCommentFormat: 'combined'
+  magicNoteCommentFormat: 'combined',
+  magicNoteCanvasPageCount: 1
 }
 
 export class ApplicationSettingsStore {
@@ -286,8 +287,10 @@ export class ApplicationSettingsStore {
       | 'version'
       | 'applicationNavigation'
       | 'localInferenceEnabled'
+      | 'magicNoteCanvasPageCount'
     > & {
       conversationHtmlRenderingEnabled?: boolean
+      magicNoteCanvasPageCount?: number
       version: number
     }
   ): Promise<StoredApplicationSettings> {
@@ -525,6 +528,7 @@ export class ApplicationSettingsStore {
         stored.magicNotesShowIncompleteTodoCount,
       magicNoteCommentMode: stored.magicNoteCommentMode,
       magicNoteCommentFormat: stored.magicNoteCommentFormat,
+      magicNoteCanvasPageCount: stored.magicNoteCanvasPageCount,
       ...(this.warnings.length > 0 ? { warnings: [...this.warnings] } : {})
     }
   }
@@ -563,7 +567,8 @@ export class ApplicationSettingsStore {
         magicNotesShowIncompleteTodoCount:
           next.magicNotesShowIncompleteTodoCount,
         magicNoteCommentMode: next.magicNoteCommentMode,
-        magicNoteCommentFormat: next.magicNoteCommentFormat
+        magicNoteCommentFormat: next.magicNoteCommentFormat,
+        magicNoteCanvasPageCount: next.magicNoteCanvasPageCount
       }
       for (const listener of this.listeners) listener(settings)
       return settings