Make read-receipts and view-reactions draggable-scrollable

[chrisbobbe]

For manual testing, ideally use a real device, to see the fling and drag responses faithfully to what most users will experience :)

Screenshots coming soon.

Selected commit messages:

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ead_receipts: Improve UX by making the sheet draggable-scrollable

The Figma asks that the sheet be expandable to fill the screen:
https://www.figma.com/design/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=11367-21131&m=dev
and that's implemented here.

Compare f8ddff2, where we removed an earlier implementation. I
hadn't tried to bring that back yet because I wanted to support
triggering resize from a drag handle at the top, and I couldn't
figure out how to do that. IIRC I could only get the drag handle to
respond to drag-down gestures (via enableDrag: true) by shifting
the sheet's position downward. That worked as a way to dismiss the
sheet, but it was frustratingly different from the gesture handling
on the scrollable list:

• The slide-to-dismiss looked different from the shrink-and-dismiss,
  an awkward inconsistency
• The list would respond to upward drags, too (by growing and
  showing more content)

I've managed it here, modulo with a header instead of a drag handle,
by making sure the scrollable area extends through the top of the
sheet. (Done with a CustomScrollView, pinning the header to the
viewport top.)

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emoji_reaction: Use DraggableScrollableModalBottomSheet for view-reactions

Like we did for read-receipts in a recent commit. I think this
behavior is implied in the Figma, but it's not as explicit:
https://www.figma.com/design/1JTNtYo9memgW7vV6d0ygq/Zulip-Mobile?node-id=5878-19207&m=dev
...anyway, helpful to be consistent with read-receipts, I think,
which is similarly a read-only view that might have a long list to
show.

I thought this would be more complicated than it turned out to be --
thanks to SliverSemantics, I can actually wrap the list of voters in
a labeled container node, because
DraggableScrollableModalBottomSheet's API takes a sliver for the
content.

[chrisbobbe]

Finger down on header, move up and down

Before:

ScreenRecording_08-14-2025.12-55-18_1.mov

After:

ScreenRecording_08-14-2025.12-57-41_1.mov

Finger down in list, move up and down

Before:

ScreenRecording_08-14-2025.12-56-03_1.mov

After:

ScreenRecording_08-14-2025.12-58-02_1.mov

[chrisbobbe]

I can record the view-reactions behavior too, but it's similar, and anyway the best way to see the behavior is to try it yourself :)

[rajveermalviya]

Thanks for working on this @chrisbobbe! This works great on my device, and LGTM.