fix(TUI): stop logs screen tail-follow flicker

[stablegenius49]

Summary

• disable the Log widget's built-in auto-scroll in the TUI logs screen and manage tail-following manually
• scroll to the latest line with scroll_end(animate=False) only when tail-following is enabled
• disable tail-following when the user scrolls upward, and re-enable it when they jump back to the bottom
• add unit tests covering the new tail-following behavior

Closes #803.

Testing

• PYTHONPATH=src pytest --confcutdir=tests/unit tests/unit/test_logs_screen.py
• python3 -m py_compile src/tui/screens/logs.py tests/unit/test_logs_screen.py