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/capabilities/default.json

@@ -2,9 +2,12 @@
   "$schema": "../gen/schemas/desktop-schema.json",
   "identifier": "default",
   "description": "Capability for the main window",
-  "windows": ["main"],
+  "windows": [
+    "main"
+  ],
   "permissions": [
     "core:default",
-    "opener:default"
+    "opener:default",
+    "dialog:default"
   ]
 }