feat(viewer): add optional gapless continuous page display

M3: preserve reading position while joining page edges, with persisted defaults and opt-in UI control. Translate both labels in all 22 catalogs, update attribution, and cover catalog completeness with a regression test. Rebase PR #3101 onto current main while retaining navigation and rendering fixes.

docs/page-display.md

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+# Continuous page display
+
+> **Last updated:** 2026-10-17 · Viewer: optional page gaps.
+
+To read pages as a continuous strip, open **View → Page display**, choose **Single page**,
+turn **Continuous scrolling** on, and turn **Show gaps between pages** off. The Page display
+button on the viewer rail opens the same choices. The command palette also offers
+**Show gaps between pages** as a toggle.
+
+Hiding gaps joins the displayed page edges and removes page shadows. It does not crop or
+change the PDF: white margins already in a page remain. Pages of different widths stay
+centered. In two-page view, it removes both the gutter within a row and the gap between rows.
+
+For future documents, open **Preferences → Documents and view** and uncheck
+**Show gaps between pages**. This default survives a restart. Existing documents retain their
+current view; the per-document toggle does not change the preference for future documents.
+
+Desktop launch options:
+
+```sh
+pdfcraft --layout continuous --page-gaps off document.pdf
+pdfcraft --default-page-gaps off
+```
+
+The opt-in UI control channel accepts `set key=page-gaps value=off` for the active document
+and `set key=default-page-gaps value=off` for future documents. Its `command` method accepts
+`id=view.page_gaps`, and `state` reports `active.page_gaps` and `active.continuous`.
+
+Revision history: 2026-10-17 — added local gapless-display controls and documentation.