feat: implement sync conflict resolver with timestamp-based merging

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Co-Authored-By: Claude <noreply@anthropic.com>

lib/sync-conflict-resolver.ts

@@ -0,0 +1,58 @@
+import { BibleHighlight } from '@/types'
+
+/**
+ * Resolves conflicts between client and server versions of a highlight.
+ * Uses timestamp-based "last write wins" strategy.
+ */
+export function resolveConflict(
+  clientVersion: BibleHighlight,
+  serverVersion: BibleHighlight
+): BibleHighlight {
+  // Use timestamp to determine which version is newer
+  const isServerNewer = serverVersion.updatedAt > clientVersion.updatedAt
+
+  // Take the newer version and mark as synced
+  const resolvedVersion = isServerNewer ? serverVersion : clientVersion
+
+  return {
+    ...resolvedVersion,
+    syncStatus: 'synced' as const
+  }
+}
+
+/**
+ * Merges server highlights with client highlights.
+ * - Adds new highlights from server
+ * - Updates existing highlights if server version is newer
+ * - Keeps client highlights if client version is newer
+ */
+export function mergeHighlights(
+  clientHighlights: BibleHighlight[],
+  serverHighlights: BibleHighlight[]
+): BibleHighlight[] {
+  const clientMap = new Map(clientHighlights.map(h => [h.id, h]))
+  const serverMap = new Map(serverHighlights.map(h => [h.id, h]))
+
+  const merged = new Map<string, BibleHighlight>()
+
+  // Add all client highlights, resolving conflicts with server
+  for (const [id, clientH] of clientMap) {
+    const serverH = serverMap.get(id)
+    if (serverH) {
+      // Conflict: both have this highlight
+      merged.set(id, resolveConflict(clientH, serverH))
+    } else {
+      // No conflict: only client has it
+      merged.set(id, clientH)
+    }
+  }
+
+  // Add any server highlights not in client
+  for (const [id, serverH] of serverMap) {
+    if (!clientMap.has(id)) {
+      merged.set(id, { ...serverH, syncStatus: 'synced' as const })
+    }
+  }
+
+  return Array.from(merged.values())
+}