Protocol 12: Up next (a shared queue) and where the video comes from

Up next: a server-side queue anyone edits by item id (add at end or
next, move, remove, play now, skip); each item says who added it and
how it plays. At the real end the first item swaps in after the end
card's countdown, put up by whoever can (its adder for through-me links
and files, someone with access for Jellyfin, else anyone); items that
can't play are skipped with the reason. The next item's link is
resolved ahead. Greyed "Continues with" episodes below. A chip with the
controls opens it as a popover (leaf_widgets, local path dependency for
now); "Add to queue" next to Put it up everywhere.

Delivery: the server measures the source's rate, the media's bitrate
and each member's read rate; the members card shows a source line
(who passes it on, through what, keeping up or not, source left) and
each member's route and buffer. A slow source is told once per media.
A failed server fetch is announced to everyone.

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

apps/desktop/lib/src/shell/handoff.dart

@@ -9,6 +9,11 @@ import '../files/ui/file_tiles.dart' show baseName;
 // field: what it is ([classifyHandoff], pure) and the small card that asks
 // how to open it ([HandoffCard]). The shell wires both.
 
+/// Open overlays over the shell that take drops and pastes themselves
+/// (the Up next popover, which is not a route): the shell's own handoff
+/// stays off while any is open.
+int handoffHolds = 0;
+
 /// What was dropped or pasted.
 sealed class Handoff {
   const Handoff();