feat: respect existing zap tags in source content when creating highlights

- Updated addZapTags function to check for existing zap tags in source event
- When source has zap tags (author group), split proportionally:
  - Highlighter gets their configured percentage
  - Remaining percentage distributed among existing authors proportionally
- Example: 50/50 split with 2 source authors = 50% highlighter, 25% each author
- Falls back to simple two-way split if no existing zap tags
- Prevents duplicate entries if highlighter is already in author group

.cursor/rules/zaps-and-zap-splits.mdc

@@ -0,0 +1,20 @@
+---
+description: Specification for zaps and zaps splits
+alwaysApply: false
+---
+
+When we create highlights, we want to add `zap` tags to the event, to allow for value splits between the highlighter/curator and the author (or authors).
+
+`zap` tags are defined in Appendix G of NIP-57:
+- https://github.com/nostr-protocol/nips/blob/master/57.md
+
+More on `zap` tags here:
+- https://nostrbook.dev/tags/zap
+
+Note that nostr-native content might have `zap` tags already, which can be seen as the "author group" of e.g. the long-form article (writer, editor, designer, etc). We should respect these `zap` tags and include them into our "zap splits" appropriately.
+
+Example: if our zap-split setting is 50/50, and the nostr-native blog post has two authors, our zap splits should be as follows:
+
+- Highlighter: 50%
+- Author1: 25%
+- Author2: 25%

src/services/highlightCreationService.ts

@@ -77,7 +77,7 @@ export async function createHighlight(
   // Add zap tags for nostr-native content (NIP-57 Appendix G)
   if (typeof source === 'object' && 'kind' in source) {
     const zapSplitPercentage = settings?.zapSplitPercentage ?? 50
-    addZapTags(highlightEvent, account.pubkey, source.pubkey, zapSplitPercentage)
+    addZapTags(highlightEvent, account.pubkey, source, zapSplitPercentage)
   }
 
   // Sign the event
@@ -186,31 +186,67 @@ function extractContext(selectedText: string, articleContent: string): string |
 
 /**
  * Adds zap tags to a highlight event for split payments (NIP-57 Appendix G)
+ * Respects existing zap tags in the source event (author group)
  * @param event The highlight event to add zap tags to
  * @param highlighterPubkey The pubkey of the user creating the highlight
- * @param authorPubkey The pubkey of the original article author
+ * @param sourceEvent The source event (may contain existing zap tags)
  * @param highlighterPercentage Percentage (0-100) to give to the highlighter (default 50)
  */
 function addZapTags(
   event: NostrEvent,
   highlighterPubkey: string,
-  authorPubkey: string,
+  sourceEvent: NostrEvent,
   highlighterPercentage: number = 50
 ): void {
-  // Calculate weights based on percentage
-  // Using simple integer weights where highlighterPercentage:authorPercentage ratio is maintained
-  const highlighterWeight = Math.round(highlighterPercentage)
-  const authorWeight = Math.round(100 - highlighterPercentage)
-  
   // Use a reliable relay for zap metadata lookup (first non-local relay)
   const zapRelay = RELAYS.find(r => !r.includes('localhost')) || RELAYS[0]
   
+  // Extract existing zap tags from source event (the "author group")
+  const existingZapTags = sourceEvent.tags.filter(tag => tag[0] === 'zap')
+  
+  if (existingZapTags.length > 0) {
+    // Calculate total weight from existing zap tags
+    const totalExistingWeight = existingZapTags.reduce((sum, tag) => {
+      const weight = parseInt(tag[3] || '1', 10)
+      return sum + weight
+    }, 0)
+    
+    // Calculate author group percentage (what remains after highlighter's share)
+    const authorGroupPercentage = 100 - highlighterPercentage
+    
     // Add zap tag for the highlighter
-  event.tags.push(['zap', highlighterPubkey, zapRelay, highlighterWeight.toString()])
+    event.tags.push(['zap', highlighterPubkey, zapRelay, highlighterPercentage.toString()])
+    
+    // Add proportionally adjusted zap tags for each existing author
+    // Don't add the highlighter again if they're already in the author group
+    for (const zapTag of existingZapTags) {
+      const authorPubkey = zapTag[1]
+      
+      // Skip if this is the highlighter (they already have their share)
+      if (authorPubkey === highlighterPubkey) continue
+      
+      const originalWeight = parseInt(zapTag[3] || '1', 10)
+      const originalRelay = zapTag[2] || zapRelay
+      
+      // Calculate proportional weight: (original weight / total weight) * author group percentage
+      const adjustedWeight = Math.round((originalWeight / totalExistingWeight) * authorGroupPercentage)
+      
+      // Only add if weight is greater than 0
+      if (adjustedWeight > 0) {
+        event.tags.push(['zap', authorPubkey, originalRelay, adjustedWeight.toString()])
+      }
+    }
+  } else {
+    // No existing zap tags, use simple two-way split between highlighter and source author
+    const authorWeight = Math.round(100 - highlighterPercentage)
+    
+    // Add zap tag for the highlighter
+    event.tags.push(['zap', highlighterPubkey, zapRelay, highlighterPercentage.toString()])
     
     // Add zap tag for the original author (only if different from highlighter)
-  if (authorPubkey !== highlighterPubkey) {
-    event.tags.push(['zap', authorPubkey, zapRelay, authorWeight.toString()])
+    if (sourceEvent.pubkey !== highlighterPubkey && authorWeight > 0) {
+      event.tags.push(['zap', sourceEvent.pubkey, zapRelay, authorWeight.toString()])
+    }
   }
 }