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。

FEATURES.md

@@ -643,7 +643,9 @@ records are listed separately and do not introduce another feature status.
   is per annotation page or Quill mode, not global document history; there is no
   infinite canvas or PNG download button. Drafts, saved entries and canvas-source
   todos use the default model's image-input capability for page images plus text,
-  or an explicitly labelled text-only fallback. Purely visual entries/drafts
+  or an explicitly labelled text-only fallback. Canvas pages to send selects the
+  first 1-8 pages in current order (default 1) for both text and images; saving and
+  exporting still support 50 pages. Purely visual entries/drafts
   require an image-capable model. Immediate-comment mode offers manual canvas
   analysis; after-save automatic analysis failures do not block or undo saving.
   Text-only comments survive unchanged extracted text; visual comments are