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/tauri.conf.json

@@ -6,7 +6,7 @@
   "build": {
     "beforeDevCommand": "npm run dev",
     "devUrl": "http://localhost:1420",
-    "beforeBuildCommand": "npm run build",
+    "beforeBuildCommand": "node ../scripts/verify-binaries.js && npm run build",
     "frontendDist": "../dist"
   },
   "app": {
@@ -38,6 +38,12 @@
   "bundle": {
     "active": true,
     "targets": "all",
+    "externalBin": [
+      "binaries/yt-dlp",
+      "binaries/aria2c",
+      "binaries/ffmpeg",
+      "binaries/deno"
+    ],
     "icon": [
       "icons/32x32.png",
       "icons/128x128.png",