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/image_cache.rs

@@ -134,47 +134,27 @@ fn serve_file(request: webkit6::URISchemeRequest, path: PathBuf) {
     }
 }
 
-/// Download all remote images in every article and rewrite their src attributes
-/// to feedthemonkey-img:// URIs so images are served through the cache handler,
-/// which re-downloads automatically on a cache miss.
-pub async fn process(articles: Vec<Article>) -> Vec<Article> {
-    let dir = images_dir();
-    std::fs::create_dir_all(&dir).ok();
-
-    let client = reqwest::Client::builder()
-        .timeout(std::time::Duration::from_secs(30))
-        .build()
-        .unwrap_or_default();
-
+/// Rewrite all remote image src attributes to feedthemonkey-img:// URIs.
+/// No network requests are made here — images are downloaded lazily by the
+/// URI scheme handler the first time the WebView requests them, then cached.
+pub fn process(articles: Vec<Article>) -> Vec<Article> {
     let re = regex::Regex::new(r#"src="(https?://[^"]+)""#).unwrap();
-
-    let mut out = Vec::with_capacity(articles.len());
-    for mut article in articles {
+    articles
+        .into_iter()
+        .map(|mut article| {
             let content = article.content.clone();
             let mut rewritten = content.clone();
-
             for cap in re.captures_iter(&content) {
                 let url = &cap[1];
-            let path = dir.join(url_to_filename(url));
-            if !path.exists() {
-                if let Ok(resp) = client.get(url).send().await {
-                    if let Ok(bytes) = resp.bytes().await {
-                        std::fs::write(&path, &bytes).ok();
-                    }
-                }
-            }
-            // Always rewrite to the scheme URI so the handler can re-download
-            // if the cache directory is ever deleted.
                 rewritten = rewritten.replace(
                     &format!("src=\"{}\"", url),
                     &format!("src=\"{}\"", original_url_to_scheme_uri(url)),
                 );
             }
-
             article.content = rewritten;
-        out.push(article);
-    }
-    out
+            article
+        })
+        .collect()
 }
 
 /// Remove cached image files no longer referenced by any article.

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
                     };