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

[bmc08gt]

PR 1 of the chat polish brief (reaction strip). Android's side is code-payments/code-android-app#1605.

Reaction strip

• Fades follow the scroll position instead of a fixed CAGradientLayer mask (8pt leading, 47pt trailing), which faded the first emoji at rest. The leading fade runs 20pt in from the edge, only while the row can scroll back. The trailing fade runs from the middle of the "+" to 14pt past it, only while the row can scroll forward, and the row is hidden from there to the strip's end. Both grow in over the first 20pt of scroll and mirror in RTL.
• Centering: each emoji is drawn into an image cropped to its ink bounds and centered in its 40pt circle, so the glyph no longer sits on the label's line box. The "+" is 40pt (was 38).
• End padding is inset + "+" + item spacing, 52pt (was 74).

Placement

The strip always opens above the bubble; the below branch in presentReactionStrip is gone. For a row that offers the strip, preview(for:raising:) returns a clear box 55 + 16pt taller than the bubble with a raised copy of the bubble at its bottom, and the preview's visiblePath covers that headroom. UIKit keeps the whole box on screen and places the menu clear of it, so a bubble at the top of the transcript moves down far enough for the strip. liftedBubbleFrame() finds where UIKit put the box, since the lift is drawn through a portal and the box's own frame stays at its origin.

The group invite card now draws its own Color.backgroundMain ground. It used to borrow the transcript's, so its lifted copy went black over the menu's dim.

Double-tap

Double-tapping any bubble with offersReactionStrip presents the strip alone, with no context menu: text bubbles, cash cards, link cards, and the quote panel inside a reply. It uses UITapGestureRecognizer(numberOfTapsRequired: 2), and the double tap only receives touches on bubbles that can take it, so a tap elsewhere doesn't wait out the timeout. The strip-only path (beginStripLift/endStripLift) holds the transcript the same way the context menu does.

On cards and quote panels, the single tap now waits out the double-tap timeout before it opens the cash detail, the link, or jumps to the quoted message. The cash card's tap moved off collection view selection onto its own recognizer, because a selection can't require another recognizer to fail. The group card's "View" button keeps its instant tap and takes no double tap; a double tap on the card around it presents the strip. Reaction pills keep their instant tap.

This matches Android's #1605, which changed A5's "link and cash cards keep their instant tap".

"+" frost and haptics

The emoji scroll on under the "+", which frosts them. Below iOS 26 that's a dark blur tinted toward the strip surface (#303030 at 0.6) under the white 0.18 fill; the iOS 26 glass button frosts on its own. A pill's long-press, which opens the reactors, plays a haptic. The strip's emoji and "+", and the pill row's "N more" and "+", do nothing on long-press and stay silent.

Constants

Identical to Android's #1605:

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

The six default emoji don't fit in 313pt: they need 8 + 6×40 + 5×4 + 4 + 40 + 8 = 320pt. At rest all six are fully opaque, but the sixth sits flush against the "+". The width is unchanged here, as on Android; this needs a design call.