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。

docs/features/magic-notes/README.md

@@ -61,8 +61,11 @@ changing saved content or export dimensions; see the
 [canvas zoom reference](../../../src/renderer/src/magic-canvas/README.md#view-zoom).
 
 Drafts, saved entries and todos sourced from canvases support AI comments using
-the default model's image-input capability. Image-capable models receive complete
-page captures and extracted text. Text-only models receive an explicitly labelled
+the default model's image-input capability. The Canvas pages to send setting
+selects the first 1-8 pages in current canvas order, defaulting to 1. It counts
+pages in one canvas, not entries, and leaves full-canvas saving and export intact.
+Image-capable models receive complete captures and extracted text for that prefix.
+Text-only models receive the same page range as an explicitly labelled
 text fallback; a purely visual canvas entry or draft reports that an image-capable
 default model is required. Canvas comments are requested manually in `immediate`
 mode, or after a manual save in `after-save-auto` mode. Analysis failure does not