fix(chat): polish the quick reaction strip and add double-tap tapback

[bmc08gt]

PR 1 of the chat polish brief (reaction strip), matched to iOS, plus two backdrop gaps and a half-sheet bug found while testing it.

Reaction strip

• Fades follow the scroll position instead of a fixed gradient. The leading fade runs 20dp in from the edge. The emoji scroll on under the "+", which frosts them with a Haze blur; the trailing fade runs from the middle of the "+" to 14dp past it, where the 40dp circle is last as tall as a 28dp emoji, and everything from there to the strip's end is hidden. Ending it later let emoji poke out around the circle's curve. Both fades mirror in RTL.
• Centering: emoji line height equals the font size, with LineHeightStyle(Center, Trim.Both) and no font padding, so glyphs sit centered in their circles. The "+" is 40dp.
• End padding is Inset + AddSize + ItemSpacing, 52dp (was 74dp).
• Placement: the strip always opens above the bubble. It only goes below for a bubble taller than the viewport. Focusing a message that can take reactions scrolls an extra 55 + 16 + 8dp so the strip has room under the top bar.

Final constants, identical to iOS:

Constant	Value
Strip height	55dp
Max width	313dp
Item size	40dp
Item spacing	4dp
Inset	8dp
"+" size	40dp
Leading fade	20dp
Trailing fade	14dp, from the middle of the "+"
Gap to bubble	16dp

The old FadeLead (28dp) is gone.

The six default emoji don't fit in 313dp: they need 8 + 6×40 + 5×4 + 4 + 40 + 8 = 320dp. At rest the strip scrolls 7dp and the last emoji sits under a partial trailing fade. The width is unchanged here; iOS has the same shortfall with the same constants, so this needs a design call.

Double-tap tapback

Double-tapping a bubble that can take reactions presents the strip alone: no selection bar, no actions. It's a new PresentReactionStrip action with a strip-only flag in ChatViewModel, separate from ToggleSelection.

This goes further than spec A5: cash bubbles, link cards and reply quotes take double-tap too. The cost is that their single tap now waits out the double-tap timeout (about 300ms) before opening the cash detail, following the link or jumping to the quote. Buttons inside cards, like "View" and "Invite People", stay instant. The iOS session has been told about the change so both platforms match.

Backdrop

While a message is raised, the composer, the $ button and the items above the oldest message (info and invite cards, the date, the hidden-messages placeholder) drew over the backdrop. They now blur and dim with the transcript. Tapping the dimmed composer clears the selection. These gaps are on code/cash too.

Half sheets

The reactors sheet couldn't stay expanded. A half sheet only offers the expanded detent while its content overflows. Five reactors overflowed at half height but fit when expanded, so expanding withdrew the detent and the sheet snapped back to half. ModalBottomSheetSceneStrategy now keeps the expanded detent once the content has overflowed. This applies to every sheet using AllowSheetExpansionWhenScrollable.

Long-press haptic

A long-press on a message buzzed twice: combinedClickable plays its own long-press haptic, and select also calls vibrator.tick(). Every combinedClickable that reports select now passes hapticFeedbackEnabled = false, so only the tick plays.

Long-presses that do nothing (the strip's emoji and "+", and the pill row's "N more" and "+") no longer play the platform haptic either. A pill's own long-press opens the reactors sheet and keeps its haptic.