wasm support

[QuartzLibrary]

No description provided.

[l1n]

Verified — this does everything we need. I built our production wasm-bindgen wrapper (parses raw API JSON into Post/Reply, discovers URLs via Thread::icons() + image_urls(), feeds caller-fetched bytes as InternedImages into to_epub_from_images) against this branch with zero source changes, ran it over a real thread with all icons interned, and the output epub is byte-identical to what our production build (from #65) serves. Keeping the async api/intern_images path alive on wasm via reqwest's fetch backend is a nicer design than my module gating, and the .cargo/config.toml rustflags are a good touch.

Happy to see this merged in place of #65 — and if it makes it to a crates.io release, we can drop our vendored copy entirely. Thanks for picking it up!