feature: cache article images for offline reading

After fetching articles, all remote images referenced in article content
are downloaded to ~/.local/share/net.jeena.FeedTheMonkey/images/ and
their src attributes rewritten to file:// URIs. Subsequent loads of the
same article (including from the cache on the next startup) display
images without a network connection.

Metered-connection awareness: image caching is skipped automatically
when GIO reports the network connection as metered, regardless of the
preference setting.

A "Cache Images" toggle in Preferences lets the user disable caching
entirely (stored in the cache-images GSettings key).

After each refresh, images no longer referenced by any article in the
current unread list are deleted from the cache directory to prevent
unbounded disk growth.

src/main.rs

@@ -1,6 +1,7 @@
 mod api;
 mod app;
 mod cache;
+mod image_cache;
 mod filters;
 mod preferences_dialog;
 mod article_row;