Moments across up to four pages: stacked in Flow, side by side in Page/Book (BL-0055)

The save sheet gets "+ Previous page" / "+ Next page" (right to left for
rtl in Page/Book) for a picture that runs over several pages. Moments
store a layout (row | column); pages the selected region does not touch
are dropped. Crops, viewer and gallery draw both layouts.

Dialogs now clean up directly instead of waiting for the "close" event,
which a backgrounded page can deliver late (a closed sheet stayed in the
page and M stopped working).

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

public/app.css

@@ -564,3 +564,4 @@ body.nobars #bot { transform: translateY(100%); opacity: 0; pointer-events: none
 kbd { display: inline-block; min-width: 24px; padding: 1px 7px; border: 1px solid var(--line); border-bottom-width: 2px; border-radius: 5px;
   background: var(--card2); font: 13px/1.5 ui-monospace, monospace; text-align: center; }
 @media (max-width: 480px) { .moment-dialog { padding: 12px; } .moment-actions .moment-hint { flex-basis: 100%; } .moment-actions .moment-hint:empty { display: none; } }
+.moment-extend { display: flex; flex-wrap: wrap; justify-content: center; gap: 8px; margin-bottom: 8px; }