Add Finder reveal and copy-path actions for downloads

[num13ru]

Summary

• add o to reveal the selected download in Finder on macOS
• add y to copy the selected download's saved path to the terminal clipboard using OSC 52
• wrap OSC 52 sequences for tmux and GNU Screen
• show the new shortcuts in the downloads view
• report missing paths and action failures in the TUI

Other changes

• document how to run and build tork from source
• ignore binaries built under ./bin

Testing

• go test ./internal/tui
• added a test for generating an OSC 52 sequence containing the full download path

I'm open to alternative approaches if they fit the project better

[melqtx]

thanks for the pr, it looks good me, ill merge it shortly.