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/application-settings-contracts.ts

@@ -26,6 +26,7 @@ export const magicNoteCommentModeSchema = z.enum([
 ])
 
 export type MagicNoteCommentMode = z.infer<typeof magicNoteCommentModeSchema>
+export const magicNoteCanvasPageCountSchema = z.number().int().min(1).max(8)
 
 export const updateSourceSchema = z.enum(['github', 'mirror'])
 export type UpdateSource = z.infer<typeof updateSourceSchema>
@@ -59,7 +60,8 @@ const applicationPreferencesSchema = z
     magicNotesEnabled: z.boolean().default(true),
     magicNotesShowIncompleteTodoCount: z.boolean().default(true),
     magicNoteCommentMode: magicNoteCommentModeSchema.default('immediate'),
-    magicNoteCommentFormat: magicNoteCommentFormatSchema.default('combined')
+    magicNoteCommentFormat: magicNoteCommentFormatSchema.default('combined'),
+    magicNoteCanvasPageCount: magicNoteCanvasPageCountSchema.default(1)
   })
   .strict()
 
@@ -77,7 +79,8 @@ export const applicationSettingsUpdateSchema = applicationPreferencesSchema
     magicNotesEnabled: z.boolean().optional(),
     magicNotesShowIncompleteTodoCount: z.boolean().optional(),
     magicNoteCommentMode: magicNoteCommentModeSchema.optional(),
-    magicNoteCommentFormat: magicNoteCommentFormatSchema.optional()
+    magicNoteCommentFormat: magicNoteCommentFormatSchema.optional(),
+    magicNoteCanvasPageCount: magicNoteCanvasPageCountSchema.optional()
   })
   .refine((input) => Object.keys(input).length > 0, {
     message: 'At least one application setting is required'