feat: Tauri commands, concurrent refresh, download manager

Refresh fetches 8 channel feeds concurrently and mirrors thumbnails to
disk so the offline feed still renders. Downloads are capped at 2 and
shell out to yt-dlp with an absolute binary path, since GUI apps do not
inherit a login shell PATH.

src-tauri/src/thumbs.rs

@@ -0,0 +1,55 @@
+//! Local thumbnail cache.
+//!
+//! Feed thumbnails are remote `ytimg.com` URLs. Offline — the whole point of
+//! this app — those fail to load and the feed becomes a wall of broken images.
+//! So every thumbnail we learn about gets mirrored to disk.
+
+use std::path::{Path, PathBuf};
+
+pub fn cache_dir(app_data: &Path) -> PathBuf {
+    app_data.join("thumbs")
+}
+
+/// Downloads one thumbnail if it isn't already cached. Returns its path.
+pub async fn cache_one(
+    client: &reqwest::Client,
+    video_id: &str,
+    url: &str,
+    dir: &Path,
+) -> Result<PathBuf, String> {
+    let dest = dir.join(format!("{video_id}.jpg"));
+    if dest.exists() {
+        return Ok(dest);
+    }
+
+    tokio::fs::create_dir_all(dir)
+        .await
+        .map_err(|e| format!("Cannot create thumbnail cache dir: {e}"))?;
+
+    let resp = client
+        .get(url)
+        .send()
+        .await
+        .map_err(|e| format!("Thumbnail request failed: {e}"))?;
+
+    if !resp.status().is_success() {
+        return Err(format!("Thumbnail returned HTTP {}", resp.status()));
+    }
+
+    let bytes = resp
+        .bytes()
+        .await
+        .map_err(|e| format!("Cannot read thumbnail body: {e}"))?;
+
+    // Write to a temp name then rename, so an interrupted download never leaves
+    // a half-written JPEG that later looks cached.
+    let tmp = dir.join(format!("{video_id}.jpg.part"));
+    tokio::fs::write(&tmp, &bytes)
+        .await
+        .map_err(|e| format!("Cannot write thumbnail: {e}"))?;
+    tokio::fs::rename(&tmp, &dest)
+        .await
+        .map_err(|e| format!("Cannot finalize thumbnail: {e}"))?;
+
+    Ok(dest)
+}