Pages panel: keep a short page current; context menu and drag to reorder (#573)

* View: the middle button pans instead of drawing with markup tools

egui's drag responses accept every pointer button, so on Windows and macOS a
middle-button drag reached the page tools: with Draw (ink) selected it drew a
stroke, and other tools cropped, selected or moved. Linux was unaffected
because its middle-click auto-scroll already blocks page input.

Outside Linux a middle-button drag that starts in the viewport (document or
Organize Pages) now pans 1:1 while the button is held, with a Grabbing
cursor, and page input is blocked from the press through the release. Linux
keeps its latched auto-scroll.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* M3: a short page gone to from the Pages panel stays current

The current page was whichever page showed most of itself. Going to a short
page (a cheque) above a tall one scrolls it to the top, where the tall page
below shows more, so the current page and the Pages panel's highlight moved
to the tall page instead of the one clicked. The page navigated to now stays
current while it is wholly on screen.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* M4: page operations from the Pages panel: context menu and drag to reorder

Right-clicking a thumbnail offers Extract pages, Cut, Copy and Paste after, on
the selection or on the page clicked (which becomes the selection), as the
organize grid does. Dragging thumbnails moves the selection, or the page
grabbed, to the gap it is dropped on: a bar marks the gap, the page count
follows the pointer, and the panel scrolls near its edges. The move is one
undoable step; the moved pages stay selected and the current page follows
its page. Documents that forbid assembly keep click-only thumbnails.

Parity: view.thumbnail-context-ops is shipped, with what it still lacks.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* Pages panel: only the primary button drags thumbnails; a drag left over after the panel closed or the document changed is dropped

---------

Co-authored-by: Cody Vogt <Cody.Vogt@caltechsurveys.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
Co-authored-by: William Max & Co. <info@williammax.com>
Co-authored-by: Brandon Thomas <echelon@gmail.com>

parity/acrobat-features.toml

@@ -1235,11 +1235,11 @@ title = "Page operations from the thumbnails panel (drag reorder, context menu)"
 area = "B"
 tier = "P1"
 milestone = "M4"
-status = "planned"
+status = "shipped"
 commands = []
 tools = []
-evidence = []
-notes = ""
+evidence = ["crates/ui-egui/tests/editing.rs::dragging_pages_panel_thumbnails_reorders_pages", "crates/ui-egui/tests/viewer.rs::pages_panel_thumbnails_have_a_context_menu"]
+notes = "Drag the selection (or the page grabbed) to reorder, with a drop bar and edge scrolling; right-click offers Extract pages, Cut, Copy and Paste after. Not yet: Rotate, Delete, Insert and Page labels in the menu, dragging pages between documents, or dropping files onto the panel."
 
 [[feature]]
 id = "view.bookmarks-panel"