Fix intermittent image loads on rate-limited CDNs (#435)

#439 in DioxusLabs/blitz — merged 2026-05-13

Caps per-host concurrency at 6 with 429/503 retry-and-backoff (honoring Retry-After), updates the User-Agent per Wikimedia policy, and surfaces image decode/HTTP errors that were previously swallowed — together fixing intermittent image load failures on rate-limited CDNs. The browser app also gains an optional CLI URL argument for quick reproduction.

Fixes #435.