refactor: DRY up highlight classification and URL normalization

- Extract classifyHighlight and classifyHighlights utilities
- Remove duplicate highlight classification logic from 3 locations
- Use existing normalizeUrl from urlHelpers instead of duplicating
- Reduce code duplication while maintaining functionality

src/utils/highlightClassification.ts

@@ -0,0 +1,34 @@
+import { Highlight } from '../types/highlights'
+
+export type HighlightLevel = 'mine' | 'friends' | 'nostrverse'
+
+/**
+ * Classify a highlight based on the current user and their followed pubkeys
+ */
+export function classifyHighlight(
+  highlight: Highlight,
+  currentUserPubkey?: string,
+  followedPubkeys: Set<string> = new Set()
+): Highlight & { level: HighlightLevel } {
+  let level: HighlightLevel = 'nostrverse'
+  
+  if (highlight.pubkey === currentUserPubkey) {
+    level = 'mine'
+  } else if (followedPubkeys.has(highlight.pubkey)) {
+    level = 'friends'
+  }
+  
+  return { ...highlight, level }
+}
+
+/**
+ * Classify an array of highlights
+ */
+export function classifyHighlights(
+  highlights: Highlight[],
+  currentUserPubkey?: string,
+  followedPubkeys: Set<string> = new Set()
+): Array<Highlight & { level: HighlightLevel }> {
+  return highlights.map(h => classifyHighlight(h, currentUserPubkey, followedPubkeys))
+}
+