feat(word): add visual pageless view mode

[juliusknorr]

Pageless view for the Word editor (issue #2553).

Adds a per-view "pageless" flag that renders a multi-page document as one continuous sheet, without changing internal pagination. Mirrors the existing Multiple Pages view mode.

Changes

• WordControl gains isPagelessMode plus getPageGapVer()/getPageGapHor() accessors that return 0 when pageless; the inter-page gap reads in CalculateDocumentSize/OnCalculatePagesPlace now go through them, collapsing the grey gaps.
• CPage.Draw skips the per-page outline (strokeColor = undefined) when pageless.
• Public API SetPagelessMode/GetPagelessMode (modelled on Set/GetMultipageViewMode); the setter clears the page cache and re-fires layout so the outline/gap change repaints.

Notes

• View preference only — does not modify the document or affect collaborators.
• The UI toggle lives in the web-apps PR; this PR is the engine half.

True continuous reflow (future, out of scope here)

Documented now so Phase 1 doesn't paint us into a corner. The engine already has a pluggable layout architecture that is the right insertion point:

• CDocumentLayoutBase (word/Editor/Layout/Base.js) defines GetPageContentFrame / GetColumnContentFrame, whose YLimit is what forces content onto the next page.
  • Concrete layouts CDocumentPrintView and CDocumentReadView (Layout/PrintView.js, Layout/ReadView.js) are registered in CDocument at Document.js:1207-1212 (this.Layouts = {Print, Read}; this.Layout = this.Layouts.Print;) and swapped via SetDocumentReadMode (Document.js:17049`).

Phase 2 would add CDocumentPagelessView returning a near-infinite YLimit (single ever-growing page, no page breaks), register it in this.Layouts, add a SetDocumentPagelessModeswap method followingSetDocumentReadMode, and decide header/footer and section-break handling. The Phase-1 API method (SetPagelessMode`) can later route to this layout swap so the UI contract stays stable.

Deferred background colour would extend getPageBackgroundColor (api.js:14248-14254) plus a colour-picker control in ViewTab.