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/magic-notes-contracts.ts

@@ -360,6 +360,7 @@ export const magicNoteAnalysisOptionsSchema = z
     requestId: z.string().uuid(),
     direction: magicNoteCommentDirectionSchema,
     format: magicNoteCommentFormatSchema,
+    canvasPageText: z.array(z.object({ pageId: z.string().min(1), text: z.string().max(20_001) }).strict()).max(8).optional(),
     canvasImages: z.array(z.object({ pageId: z.string().min(1), dataUrl: z.string().regex(/^data:image\/(?:jpeg|png);base64,[A-Za-z0-9+/]+={0,2}$/) }).strict()).max(50).optional()
   })
   .strict()
@@ -374,8 +375,8 @@ export const magicNoteAnalyzeSchema = z
     ...magicNoteAnalysisOptionsSchema.shape
   })
   .strict()
-  .refine((input) => !input.canvasImages?.length || input.expectedRevision !== undefined, {
-    message: '画布截图必须提供记录版本', path: ['expectedRevision']
+  .refine((input) => !(input.canvasImages?.length || input.canvasPageText?.length) || input.expectedRevision !== undefined, {
+    message: '画布截图或分页文字必须提供记录版本', path: ['expectedRevision']
   })
 
 export type MagicNoteEntryAnalysisOptions = MagicNoteAnalysisOptions & { expectedRevision?: number }
@@ -394,8 +395,8 @@ export const magicTodoIdSchema = z
     ...magicNoteAnalysisOptionsSchema.shape
   })
   .strict()
-  .refine((input) => !input.canvasImages?.length || input.sourceEntryRevision !== undefined, {
-    message: '画布截图必须提供来源记录版本', path: ['sourceEntryRevision']
+  .refine((input) => !(input.canvasImages?.length || input.canvasPageText?.length) || input.sourceEntryRevision !== undefined, {
+    message: '画布截图或分页文字必须提供来源记录版本', path: ['sourceEntryRevision']
   })
 
 export type MagicTodoAnalysisOptions = MagicNoteAnalysisOptions & { sourceEntryRevision?: number }