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/pending_actions.rs

@@ -0,0 +1,65 @@
+use std::path::PathBuf;
+
+use crate::api::Api;
+
+#[derive(Debug, Clone, PartialEq, serde::Serialize, serde::Deserialize)]
+#[serde(rename_all = "lowercase")]
+pub enum Action {
+    Read,
+    Unread,
+}
+
+#[derive(Debug, Clone, serde::Serialize, serde::Deserialize)]
+pub struct PendingAction {
+    pub action: Action,
+    pub id: String,
+}
+
+fn path() -> PathBuf {
+    glib::user_cache_dir()
+        .join("net.jeena.FeedTheMonkey")
+        .join("pending_sync.json")
+}
+
+fn load() -> Vec<PendingAction> {
+    let Ok(data) = std::fs::read_to_string(path()) else { return Vec::new() };
+    serde_json::from_str(&data).unwrap_or_default()
+}
+
+fn save(actions: &[PendingAction]) {
+    let dir = path();
+    std::fs::create_dir_all(dir.parent().unwrap()).ok();
+    if let Ok(s) = serde_json::to_string(actions) {
+        std::fs::write(path(), s).ok();
+    }
+}
+
+/// Queue an action for an article. If the same article already has a pending
+/// action, it is replaced with the new one (last writer wins).
+pub fn add(action: Action, id: &str) {
+    let mut actions = load();
+    actions.retain(|a| a.id != id);
+    actions.push(PendingAction { action, id: id.to_string() });
+    save(&actions);
+}
+
+/// Send all queued actions to the server. Successfully synced actions are
+/// removed; failed ones remain in the queue for the next attempt.
+pub async fn flush(api: &Api, write_token: &str) {
+    let actions = load();
+    if actions.is_empty() {
+        return;
+    }
+
+    let mut remaining = Vec::new();
+    for pending in actions {
+        let result = match pending.action {
+            Action::Read   => api.mark_read(write_token, &pending.id).await,
+            Action::Unread => api.mark_unread(write_token, &pending.id).await,
+        };
+        if result.is_err() {
+            remaining.push(pending);
+        }
+    }
+    save(&remaining);
+}