Implement Mozilla WebExtension best practices and fix background script issues

[Copilot]

This PR implements Mozilla's WebExtension development best practices and fixes a critical issue in the background script that prevented proper functionality.

Key Changes

Fixed Critical Background Script Issue

The background script was incorrectly using window.addEventListener to handle keyboard events, which doesn't work in WebExtension background contexts. Background scripts don't have access to the DOM window object. This functionality was already properly implemented via the manifest.json commands section, so the broken code has been removed:

// ❌ Removed - doesn't work in background script context
window.addEventListener("keydown", (event) => {
  // This code never executed
});

// ✅ Already working - F9 handled via manifest commands
"commands": {
  "_execute_browser_action": {
    "suggested_key": { "default": "F9" }
  }
}

Enhanced Documentation

Completely rewrote the README.md to include:

• Development Setup: Step-by-step instructions for loading the extension in Firefox Developer Edition
• Testing Guidelines: Comprehensive testing procedures following Mozilla's recommendations
• Project Structure: Clear documentation of file organization and purpose
• Security & Privacy: Explanation of minimal permissions and privacy practices
• Contribution Guidelines: Development workflow for contributors

Mozilla Best Practices Compliance

• ✅ Uses only necessary permissions (browsingData, notifications, storage)
• ✅ Proper WebExtensions API usage throughout codebase
• ✅ Firefox Developer Edition testing workflow documented
• ✅ Security and privacy considerations documented
• ✅ No external dependencies added
• ✅ Follows official Mozilla WebExtension guidelines

Code Quality Improvements

• Removed unused customKey setting from default configuration
• Maintained proper error handling with onError functions
• Validated all JavaScript syntax and WebExtension API usage
• No breaking changes to existing functionality

The extension now fully complies with Mozilla's development standards and the keyboard shortcut (F9) works correctly through the proper WebExtension commands API.

Fixes #20.

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[Copilot]

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[TenSoja]

Approved!

Great job fixing the background script and upgrading the documentation.
This PR officially passes the “Copilot-bot review”... and just like a good Firefox extension, it didn’t crash under pressure!