Show a floating down-chevron button near the bottom of the transcript when
the user has scrolled well away from the latest message (200px hysteresis).
Clicking it re-enables sticky-bottom auto-scroll and smooth-scrolls back to
the newest message; it hides itself at the bottom and on session switch.

Reuses ChatDisplay's existing scroll state (distanceFromBottom,
isStickToBottomRef, messagesEndRef) and motion/AnimatePresence primitives.
Frontend-only; one new i18n key chat.scrollToBottom across all six locales.

Adds a reusable seed(profileDirs) hook to the e2e harness so assertions can
pre-seed an on-disk session before launch (backend-independent), and a CDP
assertion that seeds a 40-message session, opens it, scrolls up, asserts the
button appears, clicks it, and asserts the transcript returns to the bottom
and the button hides.

Resolve merge conflict in docs/loop/feature-ledger.md by unioning the
branch's ledger rows with main's reduce-motion (#51) entry, so no rows are
lost from either side. Each PR is referenced once. Locale files auto-merged
cleanly; i18n parity verified (7 locales, equal key sets, no dups).

# Conflicts:
#	docs/loop/feature-ledger.md