image-cache: rewrite URLs eagerly, download lazily via scheme handler

process() was downloading all images before returning, blocking the
article list update for potentially minutes on a first run or after a
cache wipe. Move all network I/O out of process():

- process() now only rewrites src="https://..." to the custom
  feedthemonkey-img:/// scheme — it is synchronous and instant.
- The URI scheme handler already downloads and caches on demand, so
  images are fetched the first time the WebView requests them and
  served from disk on every subsequent view.

This means the article list appears immediately after a server fetch
regardless of how many images need caching.

src/window.rs

@@ -597,7 +597,7 @@ pub mod imp {
                 async move {
                     let articles = api.fetch_unread().await?;
                     let articles = if cache_images {
-                        crate::image_cache::process(articles).await
+                        crate::image_cache::process(articles)
                     } else {
                         articles
                     };