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/tauri.conf.json

@@ -1,6 +1,6 @@
 {
   "$schema": "https://schema.tauri.app/config/2",
-  "productName": "flighttube",
+  "productName": "FlightTube",
   "version": "0.1.0",
   "identifier": "com.vincent.flighttube",
   "build": {
@@ -12,13 +12,23 @@
   "app": {
     "windows": [
       {
-        "title": "flighttube",
-        "width": 800,
-        "height": 600
+        "title": "FlightTube",
+        "width": 1280,
+        "height": 840,
+        "minWidth": 940,
+        "minHeight": 600
       }
     ],
     "security": {
-      "csp": null
+      "csp": null,
+      "assetProtocol": {
+        "enable": true,
+        "scope": [
+          "$HOME/Movies/FlightTube/**",
+          "$APPDATA/**",
+          "$APPLOCALDATA/**"
+        ]
+      }
     }
   },
   "bundle": {