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/main.rs

@@ -2,6 +2,7 @@ mod api;
 mod app;
 mod cache;
 mod image_cache;
+mod pending_actions;
 mod filters;
 mod preferences_dialog;
 mod article_row;