Add demo trio personas and polish empty-state panel UX.

Build credentials during Phase 1 and streamline human participant setup.

refactor(ui): shorten Settings menu — accordions + Model Selection merge

feat(ui): header cleanup + Table View shortcut + post-chat downloads strip

feat(ui): consolidate downloads, move theme into settings, restructure & darken settings menu, fix max-participants prop wiring

settings: drop Response Priority, make dev panel scrollable

Import LLMChats3 master as CCAI baseline