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/cache.rs

@@ -0,0 +1,29 @@
+use crate::model::Article;
+
+pub struct Cache {
+    pub articles: Vec<Article>,
+    pub selected_id: String,
+}
+
+pub fn save(articles: &[Article], selected_id: &str) {
+    let dir = glib::user_data_dir().join("net.jeena.FeedTheMonkey");
+    std::fs::create_dir_all(&dir).ok();
+    let data = serde_json::json!({
+        "articles": articles,
+        "selected_id": selected_id,
+    });
+    if let Ok(s) = serde_json::to_string(&data) {
+        std::fs::write(dir.join("cache.json"), s).ok();
+    }
+}
+
+pub fn load() -> Option<Cache> {
+    let path = glib::user_data_dir()
+        .join("net.jeena.FeedTheMonkey")
+        .join("cache.json");
+    let data = std::fs::read_to_string(path).ok()?;
+    let json: serde_json::Value = serde_json::from_str(&data).ok()?;
+    let articles: Vec<Article> = serde_json::from_value(json["articles"].clone()).ok()?;
+    let selected_id = json["selected_id"].as_str().unwrap_or("").to_string();
+    Some(Cache { articles, selected_id })
+}