Use web workers to avoid blocking the main UI thread during MSI processing

[Copilot]

Large MSI files and LZX-compressed files cause the browser to hang with "page unresponsive" warnings during parsing and extraction.

Changes

Architecture: Moved all Pyodide operations to a dedicated web worker thread

• msi_viewer_worker.js: New worker handling Pyodide initialization, MSI parsing, and file extraction
• msi_viewer.js: Refactored to communicate with worker via postMessage, handle UI updates only
• msi_viewer.md: Enhanced CSS with animated loading spinner and error states

Progress feedback: Added visual indicators for background operations

• Animated spinner during processing
• Error state styling with clear messaging
• Progress messages for each operation phase

Resilience: Exponential backoff retry logic with timeout protection

// Configuration constants for retry behavior
this.WORKER_INIT_MAX_RETRIES = 10;
this.WORKER_INIT_BASE_DELAY = 500;
this.WORKER_INIT_MAX_DELAY = 5000;
this.WORKER_INIT_BACKOFF_FACTOR = 1.5;

Security: Input validation and safe variable passing

• Table names validated via regex to prevent control characters
• Python variables set via pyodide.globals.set() instead of string interpolation
• CodeQL analysis: 0 alerts

Memory management: Timeout cleanup prevents leak accumulation

if (this._loadRetryTimeout) {
  clearTimeout(this._loadRetryTimeout);
}
this._loadRetryTimeout = setTimeout(/* ... */);

Screenshot

The UI now remains responsive during all MSI operations, with clear visual feedback showing current processing status.

Original prompt

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This section details on the original issue you should resolve

<issue_title>[Feature] Use web workers to avoid blocking the main UI thread</issue_title>
<issue_description>For extracting some MSI files (particularly ones that are large or that use LZX compression), the main UI thread for a browser tab can hang for a bit, resulting in a pop up asking if the user wants to wait. Running the Pyodide scripts in a separate thread using web workers might help avoid those pop-ups -- and some form of indicator could be added to show that work is still happening in the background.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [Feature] Use web workers to avoid blocking the main UI thread #68

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