browser-extension project: Add Fallback for Missing API Response Data

[Joaquimdacosta1999]

The displayCarbonUsage function assumes response.data.data contains carbonIntensity and fossilFuelPercentage, but missing data could cause errors. Adding checks (e.g., if (response.data.data?.carbonIntensity == null) { throw new Error('Missing carbon intensity data'); }) would prevent runtime errors and allow for graceful fallback (e.g., displaying “Data unavailable” for missing fields).