feat(engine): replace vague error strings with structured get_engine_status diagnostics

Add a single Rust command returning per-engine structured status with
resolved path, version, error context, stderr tail, and remediation hints.

- Aria2: binary check, daemon startup error, RPC readiness, live stderr capture
- yt-dlp: binary check, onedir layout validation (_internal/Python.framework)
- FFmpeg / Deno: binary check, version parse
- UI: Ready/Error badge + expandable technical detail panel
- Old test_* commands kept for backward compatibility

src/bindings/EngineStatusResult.ts

@@ -0,0 +1,4 @@
+// This file was generated by [ts-rs](https://github.com/Aleph-Alpha/ts-rs). Do not edit this file manually.
+import type { EngineStatusItem } from "./EngineStatusItem";
+
+export type EngineStatusResult = { engines: Array<EngineStatusItem>, };