fix: improve popup stability, Safari compatibility, and translation status feedback

[HirokiChin]

PR Description

First of all, thank you for open-sourcing and maintaining this incredibly practical extension. It has been very helpful.

This PR focuses on reliability and Safari compatibility improvements:

Summary

• Fixes an accidental endpoint-modal dismissal bug: when users selected text inside the modal and released the pointer outside, the overlay click handler could close the modal unexpectedly. The close logic now requires a full overlay click (pointer down + pointer up on backdrop), which prevents frequent accidental closes while selecting or dragging text.
• Adds Safari TTS compatibility support
• Updates installation docs for Safari in both README.md and README_EN.md
• Adds translation status feedback to make processing state more visible to users