fix(frontend): hide template destinations a catalog policy emptied (#14667)

Blocking starter templates left the picker advertising places with nothing
in them: every category tab still opened, onto an empty pane explaining the
policy, and the welcome overlay still offered Simple Agent, Vector Store
RAG, and Browse more whether or not those templates survived.

The nav now drops a tab once its last template goes, along with any group
heading left standing over nothing, and lands on a tab that still has
content instead of opening a blocked one. With the whole catalog blocked it
falls back to the all-templates listing, whose empty copy speaks for the
catalog rather than just the featured cards.

On the welcome overlay each quick-template button appears with its template
and Browse more with any template at all, since a button for a blocked
template only fails its lookup on click. When nothing survives the row
becomes the one action left: start from the blank flow already underneath.

A single availableTemplateTabs decides all of this, and GetStartedComponent
now shares its featured-key list and visibility filter so the cards and the
nav cannot disagree. Its own empty state goes with them — that tab is no
longer reachable with nothing to show, so the copy was unreachable.