Add a one-shot flash component used across the app — connect success,
disconnect, account delete, and registration — instead of per-page ad-hoc
markup. setFlash queues a short-lived HttpOnly+SameSite cookie carrying an
opaque code; takeFlash consumes it on the next full-page render (not on
HTMX fragments). flashBanner maps the code to a styled, role=status banner;
the message text lives server-side in flashMessages so the cookie never
carries free text and a forged/unknown code renders nothing.
Wire it into the list page (the PRG landing spot for connect/registration)
and set it on registration and connect-callback success. Styled with the
existing design-system tokens; header gains an Account nav link.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
