webclient: the push permission is asked once a session, after signing in ...

The permission was only ever asked from the switch in Settings, and a call
without a user gesture is ignored by Firefox and Safari and answered with a
quiet icon by Chrome — so the app now offers a bar whose button click is that
gesture. It is asked once per browser session (there is no marker that tells a
fresh sign-in from a reload, so the session is the honest bound), and a
refusal ends it for good: only an explicit block counts, and after that the
switch in Settings is the single way back to it.

The policy lives in useNotificationPrompt.js as a layer over usePush, which
is untouched: the granted-but-unsubscribed trap is what makes the eligibility
gate "permission is still default", otherwise the bar would silently
re-subscribe whoever turned notifications off by hand. The banner itself is
presentation only, and is the component proposed to gnexus-ui-kit
(docs/tasks/gn-permission-banner.md there) — the kit takes the gesture, the
view and the contract, never the storage keys or the timing.