feat(reactions): undo reactions/reposts, expanded catalog, adaptive recent list

[dmnyc]

Summary

Three improvements to the emoji reaction experience, built on top of the NIP-78 cross-device settings sync branch so the adaptive quick list syncs across devices automatically.

• Undo reactions — tapping the heart on a post you've already reacted to shows a "Remove Reaction" confirmation popover (matching the repost undo pattern). Confirmed removal sends a NIP-09 kind-5 deletion and removes the reaction optimistically.
• Undo repost — the repost popover shows "Undo Repost" (in red) when you've already reposted, using NIP-09 kind-5 deletion.
• Expanded emoji catalog — ~300 additional entries across all categories (Emoji 13–15: new faces, hand gestures, animals, food, activities, travel, objects, symbols).
• Adaptive recent list — any emoji you react with is automatically added to your quick-reaction picker on first use, including emojis picked from the full library. The existing frequency sort ensures often-used emojis rise to the top; long-press any cell to remove it. The quick list and frequency map are included in the NIP-78 kind-30078 settings payload so the adapted list syncs across devices.
• Reaction picker UX fixes — quick-reaction grid is capped at 18 (3 rows × 6 columns, no scrolling). "More reactions" is pinned as a centered footer row that is always visible regardless of list length. Popover arrow now matches the body material.

Base branch: feat/nip78-settings-sync — the NIP-78 sync infrastructure is a prerequisite. This PR should merge after #173.

Implementation notes

• DeletionSender.deleteById() — new helper for undo flows when only the event ID is known.
• EngagementCounts.reposterEventIds — maps pubkey → kind-6 event ID so repost undo knows which event to delete.
• Reactor.reactionEventId — populated after signing or on relay ingest for reaction undo.
• ReactionSender.clearSent() / RepostSender.clearSent() — reset local sent-state so re-reacting/reposting works after an undo.
• EmojiRepository.setFrequency(_:) — restore-only setter that skips the sync trigger.
• Reaction undo uses a dedicated showRemoveReactionMenu popover (same pattern as repost) rather than an inline button, preventing accidental dismissal from triggering removal.
• EmojiReactionPicker no longer carries a ScrollView or maxGridHeight parameter — fixed grid + pinned footer eliminates the scroll-hides-plus-button problem.

Test plan

• Tap the heart on a post → emoji picker opens. Pick an emoji → reaction appears, count increments.
• Tap the heart again on a post you've already reacted to → "Remove Reaction" popover appears; tapping it removes the reaction. Dismissing without tapping does nothing.
• Open and close the "Remove Reaction" popover repeatedly — reaction stays until explicitly confirmed.
• Long-press the repost icon on a post you've reposted → "Undo Repost" appears in red. Tap it → repost removed.
• React with an emoji not in your quick list → it auto-appears in the picker next time.
• Pick an emoji from the full library (+) and use it as a reaction → it is added to the quick list.
• Use the same emoji several times → it rises toward the top of the picker grid.
• Long-press a cell in the picker → "Remove from quick reactions" removes it.
• With more than 18 quick reactions, confirm the grid shows exactly 3 rows and "More reactions" footer is always visible without scrolling.
• Confirm the popover arrow color matches the popup body in both light and dark mode.
• Open the full emoji library (+) → confirm ~300 new emojis are present across categories.
• Sign in on a second device after reacting on the first → quick list and frequency map sync via NIP-78 (requires feat(settings): NIP-78 cross-device sync of Interface preferences #173 merged first).