Merge zaps into ReactionGroup to fix duplicate notification cards

[barrydeen]

Summary

• Merges ZapGroup into ReactionGroup using a ZAP_EMOJI sentinel key (same pattern as REPOST_EMOJI), fixing duplicate notification cards when a note receives both reactions and zaps
• Adds zapEntries: List<ZapEntry> field to ReactionGroup to preserve per-zap data (sats, message)
• Removes ZapGroup data class, ZapGroupRow composable, and all associated branching
• Zap rows render inside ReactionGroupRow in order: zaps → reposts → emoji reactions

Test plan

• Receive reactions + zaps on the same note — single card with zap rows + reaction rows
• Old zap + recent reaction — zap should NOT appear in "recent" split
• ZAPS filter tab still shows relevant notifications
• Zap amounts and messages still display correctly
• Repost + zap + reaction on same note — all three sections render in correct order