Sync browser viewport with page pagination

[Copilot]

page --next / --prev only changed the CLI-rendered slice of the snapshot; the browser itself stayed at the old scroll position. This updates pagination to move the real viewport first, then render from the refreshed page state.

• CLI pagination flow

  • page() now resolves the target page, computes its scrollTop, sends a browser scroll action, and then fetches a fresh snapshot before rendering.
  • This applies to --next, --prev, and explicit page selection.

• Protocol + extension support

  • Added a new scroll action to the shared protocol.
  • Routed scroll through the extension service worker to the content script.
  • Implemented a content-side scroll handler with window.scrollTo(...) so pagination updates the actual browser viewport.

• Focused pagination logic

  • Extracted page/scroll helpers in Rust for:
    • current page resolution
    • total page calculation
    • target scroll offset clamping on the last page

• Targeted coverage

  • Added focused Rust tests for adjacent page navigation, explicit page clamping, and maximum scroll offset handling.

let resolved_page = resolve_requested_page(&snapshot, page_num, next, prev);

if let Some(target_page) = resolved_page {
    send_ok(Request::new(
        actions::SCROLL,
        json!({
            "session_id": session_id,
            "top": scroll_top_for_page(&snapshot, target_page),
        }),
    ))
    .await?;
}

let snapshot = fetch_snapshot(session_id, actions::GET_PAGE_FRESH).await?;

• • Pagination viewport behavior example: https://github.com/user-attachments/assets/70e6f2fd-c213-419d-b9f2-56bc4f8d368d