fix: improve subtitle readability

[nightcityblade]

Fixes #324

Summary

• make browser extension captions use a larger centered YouTube-like subtitle style with per-line dark backgrounds and text shadow
• make Python client transcript output wrap to terminal width and show the latest lines as a centered subtitle block

Tests

• python -m unittest tests.test_client -v
• python -m py_compile whisper_live/utils.py
• node --check Audio-Transcription-Chrome/content.js
• node --check Audio-Transcription-Firefox/content.js
• git diff --check