fix(ytdlp): check for updates during long tray sessions

[wakamex]

Motivation

Closing YTubic hides it to the tray by default, so the process can stay alive for days and never run the launch-only yt-dlp update check. The existing 72-hour cadence can therefore be missed indefinitely. My running version used yt-dlp from August 14 and stopped working today.

Implementation

• Poll hourly while the app process is alive.
• Keep the existing Rust-side 72-hour throttle, so early polls stay local.
• Use a dedicated update-only command guarded by the same mutex as setup and retry.
• Avoid repeating readiness events, first-run setup, macOS prewarming, or legacy cleanup.

Testing

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