fix(screenshot): allow repeated annotated captures

[louzt]

Summary

• keep the screenshot lock in $XDG_RUNTIME_DIR and recover stale lockfiles by PID
• create the screenshot directory automatically before saving captures
• launch Satty without blocking the script so a new annotated capture can start while an earlier Satty window is still open

Problem

Annotated captures stayed locked for the whole Satty session, so opening a previous preview prevented taking the next screenshot.

Testing

• built the script with scriptisto build /home/lou/niri-scripts/screenshot
• validated the updated local setup on niri