Hot-link image handling — add Referer header to fetch

Some CDNs (WeChat, JianShu, etc.) reject image requests without a matching Referer header → image download fails with 403. Currently Magpie keeps the remote URL on failure; ideally we should retry with a forged Referer.

Implementation: use chrome.declarativeNetRequest to dynamically add Referer: <source_url> on image fetches. Retry once on 403.