refactor(backend): migrate to tauri v2 sidecars and resolve pipe deadlocks

- Replace tokio::process with tauri_plugin_shell sidecar API for native cross-compilation bundle execution.
- Implement non-blocking stdout/stderr multiplexing in yt-dlp spawn to prevent OS pipe buffer deadlocks.
- Refactor capabilities to restrict execution explicitly to sidecars instead of wildcards.
- Rename local binaries to strictly adhere to target-triple architecture suffixes.
- Drop manual get_binary_name path resolutions in favor of Tauri's externalBin bundler.

src-tauri/capabilities/default.json

@@ -9,6 +9,28 @@
     "opener:default",
     "dialog:default",
     "notification:default",
-    "notification:allow-is-permission-granted"
+    "notification:allow-is-permission-granted",
+    "shell:allow-execute",
+    {
+      "identifier": "shell:allow-execute",
+      "allow": [
+        {
+          "cmd": "yt-dlp",
+          "args": true
+        },
+        {
+          "cmd": "aria2c",
+          "args": true
+        },
+        {
+          "cmd": "ffmpeg",
+          "args": true
+        },
+        {
+          "cmd": "deno",
+          "args": true
+        }
+      ]
+    }
   ]
 }