feat: implement sync conflict resolver with timestamp-based merging

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

__tests__/lib/sync-conflict-resolver.test.ts

@@ -0,0 +1,75 @@
+import { resolveConflict } from '@/lib/sync-conflict-resolver'
+import { BibleHighlight } from '@/types'
+
+describe('SyncConflictResolver', () => {
+  it('should prefer server version if newer', () => {
+    const clientVersion: BibleHighlight = {
+      id: 'h-1',
+      verseId: 'v-1',
+      color: 'yellow',
+      createdAt: 1000,
+      updatedAt: 1000,
+      syncStatus: 'pending'
+    }
+
+    const serverVersion: BibleHighlight = {
+      id: 'h-1',
+      verseId: 'v-1',
+      color: 'blue',
+      createdAt: 1000,
+      updatedAt: 2000, // newer
+      syncStatus: 'synced'
+    }
+
+    const result = resolveConflict(clientVersion, serverVersion)
+    expect(result.color).toBe('blue')
+    expect(result.updatedAt).toBe(2000)
+  })
+
+  it('should prefer client version if newer', () => {
+    const clientVersion: BibleHighlight = {
+      id: 'h-1',
+      verseId: 'v-1',
+      color: 'blue',
+      createdAt: 1000,
+      updatedAt: 3000, // newer
+      syncStatus: 'pending'
+    }
+
+    const serverVersion: BibleHighlight = {
+      id: 'h-1',
+      verseId: 'v-1',
+      color: 'yellow',
+      createdAt: 1000,
+      updatedAt: 2000,
+      syncStatus: 'synced'
+    }
+
+    const result = resolveConflict(clientVersion, serverVersion)
+    expect(result.color).toBe('blue')
+    expect(result.updatedAt).toBe(3000)
+  })
+
+  it('should mark as synced after resolution', () => {
+    const clientVersion: BibleHighlight = {
+      id: 'h-1',
+      verseId: 'v-1',
+      color: 'yellow',
+      createdAt: 1000,
+      updatedAt: 2000,
+      syncStatus: 'pending'
+    }
+
+    const serverVersion: BibleHighlight = {
+      id: 'h-1',
+      verseId: 'v-1',
+      color: 'yellow',
+      createdAt: 1000,
+      updatedAt: 2000,
+      syncStatus: 'synced'
+    }
+
+    const result = resolveConflict(clientVersion, serverVersion)
+    expect(result.syncStatus).toBe('synced')
+  })
+})