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/gen/schemas/capabilities.json

@@ -1 +1 @@
-{"default":{"identifier":"default","description":"Capability for the main window","local":true,"windows":["main"],"permissions":["core:default","opener:default"]}}
+{"default":{"identifier":"default","description":"Capability for the main window","local":true,"windows":["main"],"permissions":["core:default","opener:default","dialog:default"]}}