[rabden]

Summary

• add edge auto-scroll while selecting text in chat and terminal
• preserve chat selections across virtualized transcript rows
• add a thin terminal scrollbar with click/drag support
• show the scrollbar only while hovering the terminal and subtly widen its thumb without layout reflow

Validation

• cargo check -p zeron-ui
• cargo test -p zeron-ui --lib (468 passed)
• terminal panel tests (14 passed)
• git diff --check

The local low-memory Rust build configuration changes in .gitignore and Cargo.toml were intentionally not staged or committed.

---

^{Need help on this PR? Tag @codesmith-bot with what you need. Autofix is disabled.}