feat: implement client-side sync with bulk API

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

__tests__/lib/highlight-sync-manager.test.ts

@@ -76,4 +76,31 @@ describe('HighlightSyncManager', () => {
     const syncing = await manager.getSyncingItems()
     expect(syncing.length).toBe(1)
   })
+
+  it('should perform sync and mark items as synced', async () => {
+    const highlight: BibleHighlight = {
+      id: 'h-1',
+      verseId: 'v-1',
+      color: 'yellow',
+      createdAt: Date.now(),
+      updatedAt: Date.now(),
+      syncStatus: 'pending'
+    }
+
+    await manager.queueHighlight(highlight)
+    await manager.init()
+
+    // Mock fetch
+    global.fetch = jest.fn(() =>
+      Promise.resolve({
+        ok: true,
+        json: () => Promise.resolve({ synced: 1, errors: [] })
+      })
+    ) as jest.Mock
+
+    const result = await manager.performSync()
+
+    expect(result.synced).toBe(1)
+    expect(result.errors).toBe(0)
+  })
 })