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/Cargo.toml

@@ -26,6 +26,7 @@ tauri-build = { version = "2", features = [] }
 tauri = { version = "2", features = ["macos-private-api", "tray-icon", "image-png"] }
 tauri-plugin-opener = "2"
 tauri-plugin-dialog = "2"
+tauri-plugin-shell = "2"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 tokio = { version = "1", features = ["fs", "process", "io-util", "rt", "rt-multi-thread", "macros", "sync", "time"] }