feat: add local vs GP comparison, dynamic threads, smoothed ETA, and rate-limit retries

[nileshgarva]

Summary

This PR introduces several performance and quality-of-life enhancements to the upload workflow, including automated comparison against Google Photos, dynamic thread scaling, smoothed ETA estimation, and robust network retry policies.

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Key Features Implemented

1. Compare Local vs Google Photos (Duplicate Skipping)

• Automated Indexing: On drag-and-drop, the app automatically checks files against Google Photos before queueing the upload.
• Metadata Cache: Saves Google Photos filenames in a lightweight cache file (CompareFileList.csv under config directory) for sub-second startup times.
• Differential Uploads: Automatically filters the queue to only upload missing files and logs them in missing_files.txt. Successfully uploaded files are appended to the CSV cache.
• Protobuf Parser: Built raw varint and protobuf handlers for GET_LIB_STATE and GET_LIB_PAGE_INIT calls directly in Go, ensuring no external tools or runtime python dependencies are required.

2. Real-Time Dynamic Thread Scaling

• Live Worker Adjustments: Added a Settings gear panel with an interactive range slider inside the active upload panel.
• Dynamic Goroutine Scaling:
  • Increasing threads spawns new workers to read from the upload channel instantly.
  • Decreasing threads flags excess workers to exit cleanly immediately after completing their current file.

3. Smoothed ETA Estimation

• Jitter Prevention: Calculates remaining time using an Exponential Moving Average (EMA) algorithm ($\alpha = 0.1$) updated once per second.
• Steady Countdown: Prevents countdown jumps caused by sudden throughput drops or progress jumps when files finish.

4. Rate-Limit (429) & Network Retry Backoff

• Context Plumbing: Context cancellation is wired through all comparison and token generation requests, letting the user abort active network calls and retries immediately when clicking "Cancel".
• Auto-Backoff: Implemented exponential backoff with random jitter for robust retries when encountering rate limits or 5xx server responses.

5. Windows Launch Optimization

• Built with -ldflags "-H windowsgui" flags, disabling the Command Prompt console popup on Windows to open the GUI window directly.

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Verification

• Frontend built successfully (npm run build).
• Backend compiled and packaged correctly on Windows (go build).
• Verified duplicate checking, file logging, dynamic workers addition/subtraction, and cancellation propagation.