Extract selectBracketPair, selectWordAt, and helpers into TextSelection.js
and import it in both ChatBlockDelegate.qml and ChatPane.qml. Prompt
textarea now has the same double-click behavior as chat blocks:
delimiter adjacent to cursor -> select pair content, else -> word select.
Quotes use parity to decide direction and scan document-wide, so
multi-line content between the pair is selected in full.
Levi Neely