feat: Default new users to account + plain 5h/7d usage bars

Fresh installs (and not-yet-migrated upgraders who had the legacy
"show account name" ON) now seed [account, sessionBarPlain,
weeklyBarPlain] instead of just [account], so the menu bar is useful
out of the box. Existing 1.8.x users are untouched (migration guard
returns before the seed); users who explicitly disabled the legacy
toggle still get an empty bar (intent preserved).