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/net.rs

@@ -0,0 +1,17 @@
+//! Connectivity probing.
+//!
+//! The webview's `navigator.onLine` reports link state, not reachability — it
+//! is true on a captive-portal wifi with no real internet, which is exactly the
+//! situation this app exists for. So we actually talk to YouTube.
+
+use std::time::Duration;
+
+pub async fn is_online(client: &reqwest::Client) -> bool {
+    client
+        .head("https://www.youtube.com/")
+        .timeout(Duration::from_secs(5))
+        .send()
+        .await
+        .map(|r| r.status().is_success() || r.status().is_redirection())
+        .unwrap_or(false)
+}