feat(emoji): skin-tone selector + grid scroll-cancellation in library sheet

[dmnyc]

Summary

Two related changes to the full emoji library sheet — both shippable independently of the catalog generator and the "More reactions" hit-target work.

Skin-tone selector: a single horizontal row of 6 swatches (default + 5 Fitzpatrick tones) sits between the search field and the tab strip. Tapping a swatch persists the choice in UserDefaults via EmojiSkinTonePreference and every tone-capable cell in the grid re-renders in that tone immediately. The mechanism matches what other clients do — a single global preference rather than per-pick popovers, which conflicted with Button's tap recognizer when tried first.

Grid scroll-cancellation: the emoji and custom-image cell wrappers now use .contentShape(Rectangle()).onTapGesture instead of Button { } .buttonStyle(.plain). Button inside a LazyVGrid inside a ScrollView only cancels its tap on substantial finger movement, so a quick flick to scroll the long grid was registering as a pick on release. .onTapGesture cancels on any drag past the hit-test threshold, which is what the user expects for a scroll.

SkinTone.swift introduces:

• EmojiSkinTone enum mapping each tone to its Fitzpatrick modifier codepoint, with a preview-swatch helper.
• EmojiSkinTonePreference wrapper around UserDefaults that posts a notification when the value changes so live sheets refresh.
• String.applyingSkinTone(_:) and removingSkinTones() helpers that handle both plain base emoji (append modifier) and single-human-glyph ZWJ sequences (insert modifier before the first ZWJ, with VS-16 awareness). Multi-person sequences are intentionally out of scope.
• EmojiToneCapability — curated Set<String> of tone-capable base emoji. Membership check strips VS-16 so "👋" and "👋\u{FE0F}" both match.

Files

• SkinTone.swift — new, ~135 lines
• EmojiLibrarySheet.swift — skinToneSelector row, toneAwareCell rendering, preferredTone state + notification observer; Button → .onTapGesture swap on both grids
• wisp.xcodeproj/project.pbxproj — registers SkinTone.swift

Test plan

• Library sheet shows the "Skin tone" row at the top with 6 swatches, active one outlined
• Tap the dark swatch → every hand / body / person cell re-renders in dark tone
• Dismiss + re-open → dark swatch still selected (persisted)
• Pick a toned 👍 → reaction sent with the tone applied
• Long-press grid + drag → scrolls cleanly, no accidental pick on release
• Build clean for iOS Simulator

Closes #304.