cache: store all cached data under XDG_CACHE_HOME

Both cache.json (article list) and the images directory are
regeneratable from the server, so they belong in XDG_CACHE_HOME
(~/.cache/net.jeena.FeedTheMonkey/) rather than XDG_DATA_HOME.

src/cache.rs

@@ -6,7 +6,7 @@ pub struct Cache {
 }
 
 pub fn save(articles: &[Article], selected_id: &str) {
-    let dir = glib::user_data_dir().join("net.jeena.FeedTheMonkey");
+    let dir = glib::user_cache_dir().join("net.jeena.FeedTheMonkey");
     std::fs::create_dir_all(&dir).ok();
     let data = serde_json::json!({
         "articles": articles,
@@ -18,7 +18,7 @@ pub fn save(articles: &[Article], selected_id: &str) {
 }
 
 pub fn load() -> Option<Cache> {
-    let path = glib::user_data_dir()
+    let path = glib::user_cache_dir()
         .join("net.jeena.FeedTheMonkey")
         .join("cache.json");
     let data = std::fs::read_to_string(path).ok()?;

src/image_cache.rs

@@ -5,7 +5,7 @@ use std::path::PathBuf;
 use crate::model::Article;
 
 fn images_dir() -> PathBuf {
-    glib::user_data_dir()
+    glib::user_cache_dir()
         .join("net.jeena.FeedTheMonkey")
         .join("images")
 }