fix(chat): give the reply's close button its own ground

[bmc08gt]

Follow-up to #730, which made the reply quote Liquid Glass over the transcript. That change moved the ground under the quote's dismiss control from an opaque slab to glass sampling whatever is behind it, and the control was still drawn for the slab: an 18pt .light xmark in textSecondary, the least dense thing on the bar. Its contrast now varied with which message had scrolled past.

Drawn as xmark.circle.fill instead, so the glyph carries its own ground.

The disc is a plain translucent fill (textSecondary at 35%), not a second glass layer. Nesting glass in glass muddies the layering; a control on a glass platter belongs on it, not above it. CancelEditButton in the same bar does get its own glassBackground, and that stays correct — the bar's surface under it is opaque backgroundMain, so that is one glass level rather than two.

Palette rendering rather than monochrome: a monochrome fill knocks the ✕ out as a transparent hole, which over glass fills with the transcript — the exact thing the disc is there to prevent.

SystemSymbol gains closeCircle so the strip keeps using the app's symbol vocabulary rather than a raw string. close is untouched and still used by CancelEditButton.

Both candidates were built and compared on device before picking this one; the alternative was matching CancelEditButton's 17pt semibold xmark in textMain.