fix(ytdlp): show version and last update check in About

[wakamex]

Motivation

YTubic silently manages its own yt-dlp copy, but About did not show which version was active or when it last checked for updates. That made playback regressions difficult to distinguish from an automatic binary update or a server-side YouTube change.

Implementation

• Show the managed yt-dlp version and existing persistent update-check timestamp in About.
• Read diagnostics without making a network request or starting an update.
• Bound yt-dlp --version to 45 seconds and kill it on timeout, allowing for the documented cold macOS startup.
• Reuse the existing timestamp instead of adding another diagnostic sidecar.

Testing

• cargo test --manifest-path src-tauri/Cargo.toml --lib - 21 passed
• pnpm test - 229 passed
• pnpm build