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/Cargo.toml

@@ -18,7 +18,7 @@ crate-type = ["staticlib", "cdylib", "rlib"]
 tauri-build = { version = "2", features = [] }
 
 [dependencies]
-tauri = { version = "2", features = [] }
+tauri = { version = "2", features = ["protocol-asset"] }
 tauri-plugin-opener = "2"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"