⚡ Optimize PWA downloader: parallelize downloads and fix data race

[Snider]

This PR optimizes the DownloadAndPackagePWA function in pkg/pwa/pwa.go by parallelizing the download of HTML pages and static assets. Previously, the code waited for all HTML pages to finish downloading (and extracting their assets) before starting to download the static assets listed in the manifest. This introduced unnecessary latency.

Changes:

1. Parallel Execution: Removed the wg.Wait() call between the HTML page download loop and the static asset download loop. Now both loops dispatch goroutines immediately, allowing for better concurrency.
2. Race Condition Fix: Identified and fixed a data race condition in dn.AddData(path, body). Since DataNode is not thread-safe and is now accessed concurrently by multiple goroutines, the call is now protected by the existing mu mutex.
3. Correctness: Removed the racy if !downloaded[asset] check in the second loop. The downloadAndAdd function already performs a thread-safe check for downloaded assets, ensuring correctness without the race.

Performance:

• Baseline: ~44ms per operation (simulated with 10ms network latency).
• Optimized: ~33ms per operation.
• Improvement: ~25% reduction in total time, effectively saving one round-trip of latency in this scenario.

Verification:

• Verified correctness using go test -race ./pkg/pwa/....
• Verified existing functionality with standard tests.

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PR created automatically by Jules for task 4819682732230198450 started by @Snider

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch perf-pwa-parallel-download-4819682732230198450

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