🔒 Add schema validation for import feature

[Labreo]

🎯 What: Missing schema validation in import feature (popup.js).

⚠️ Risk: Non-array or malformed data types in imported blocklist JSON files (blockedArtists, blockedSongs, blockedAlbums) could cause unexpected runtime errors during merging or pollute storage state.

🛡️ Solution: Added validateImportSchema function to ensure imported data is a valid non-null object and that any blocklist properties present are arrays before merging into extension storage. Added unit tests in test/import.test.js.

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PR created automatically by Jules for task 11195927892693643952 started by @Labreo

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