window: persist article list and open article across restarts

On shutdown the full article list (including current read/unread state)
and the ID of the open article are saved to
~/.local/share/net.jeena.FeedTheMonkey/cache.json.

On next launch:
- The cached articles are loaded into the list immediately, before any
  network request, so the sidebar is populated and the previously open
  article is visible without waiting for the server.
- The article content is injected into the WebView once its base HTML
  finishes loading (LoadEvent::Finished), avoiding a race where
  window.setArticle() did not yet exist.
- A background refresh then fetches fresh data from the server; if the
  previously open article still exists its selection is preserved,
  otherwise the first item is selected.
- Network errors during a background refresh show a toast instead of
  replacing the visible article list with an error page.

src/model.rs

@@ -3,7 +3,7 @@ use gtk4::prelude::*;
 use gtk4::subclass::prelude::*;
 use std::cell::RefCell;
 
-#[derive(Debug, Clone, Default)]
+#[derive(Debug, Clone, Default, serde::Serialize, serde::Deserialize)]
 pub struct Article {
     pub id: String,
     pub title: String,