window: prefetch images and queue offline read/unread actions

After a successful article refresh, all images referenced in article
content are downloaded in the background so articles can be read
offline. The prefetch only runs when the cache-images setting is
enabled and the connection is not metered.

Read/unread state changes that fail to reach the server (e.g. when
offline) are now persisted to a local queue in
~/.cache/net.jeena.FeedTheMonkey/pending_sync.json. The queue is
flushed at the start of the next successful fetch.

src/image_cache.rs

@@ -134,6 +134,34 @@ fn serve_file(request: webkit6::URISchemeRequest, path: PathBuf) {
     }
 }
 
+/// Prefetch all images referenced in the articles into the cache directory.
+/// Runs entirely in the background; already-cached files are skipped.
+pub async fn prefetch(articles: 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();
+
+    let re = regex::Regex::new(&format!(r#"src="{}([^"]+)""#, regex::escape(SCHEME_PREFIX))).unwrap();
+
+    for article in &articles {
+        for cap in re.captures_iter(&article.content) {
+            let original_url = percent_decode(&cap[1]);
+            let path = dir.join(url_to_filename(&original_url));
+            if !path.exists() {
+                if let Ok(resp) = client.get(&original_url).send().await {
+                    if let Ok(bytes) = resp.bytes().await {
+                        std::fs::write(&path, &bytes).ok();
+                    }
+                }
+            }
+        }
+    }
+}
+
 /// 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.