fix(ui): make the connectivity issues overlay closable

[frdm666]

Problem

When Dekaf can't reach the Pulsar broker, it shows a full-screen "There are
connectivity issues" overlay that cannot be dismissed — no close button, no Esc,
no click-outside.

That is especially painful when the cluster requires authentication and
DEKAF_DEFAULT_PULSAR_AUTH is not set: the health check fails, the overlay
appears, and it covers the credentials button — the very control needed to fix
the connection. As reported in #352, the only workaround was to delete the
overlay via browser devtools.

What changed

• The overlay now reuses ModalElement from Modals.tsx — same backdrop, card,
  title bar with the close icon, and Esc handling as every other dialog. The
  component was already presentational; it just wasn't exported.
• The overlay is shown again when the connection drops after having been
  restored, so a new outage is still reported.
• Added a hint about setting Pulsar credentials via the 🔑 button in the
  navigation sidebar, so the dialog says what to do.
• ModalStackEntry gained an optional testId (defaults to "modal") so the
  overlay keeps the health-overlay test id introduced in e2e test suite + bunch of bugfixes #351.

The credentials editor can't be opened from the overlay directly, because
HealthCheckContext sits above Modals.DefaultProvider in the tree — hence the
textual hint.

Test plan

Added NAV-11 to e2e/src/test/scala/routes/HealthCheckSpec.scala: it aborts the
health check poll, waits for the overlay, dismisses it with Esc, then waits longer
than the 5s poll interval to assert it does not come back while connectivity is
still down, and that the credentials button is reachable again.

Both NAV-10 (existing) and NAV-11 pass; NAV-11 fails without this change.

Fixes #353

[visortelle]

@frdm666 thank you for the contribution.

We could reuse the same modal dialog component, which is used for other similar overlays. Or at least same cross icon, instead of the custom "Close" button.
It would be more consistent.

[frdm666]

Good point -done. ModalElement in Modals.tsx was already the presentational
half of the dialog (backdrop, card, title bar with the close icon, Esc handling),
it just wasn't exported. It's now exported and reused here, so the overlay looks
and behaves like every other dialog. The custom "Close" button and the manual Esc
handler are gone.

One small addition: ModalStackEntry got an optional testId (defaults to
"modal"), so the overlay keeps the health-overlay test id introduced in #351.
Wrapping the modal in a tagged <div> doesn't work -the wrapper collapses to
zero height, and NAV-10 starts failing on "hidden".

Also rebased onto main to resolve the conflict with #351, and moved the test into
HealthCheckSpec as NAV-11 (dismiss while connectivity is still down).
Both NAV-10 and NAV-11 pass; NAV-11 fails without the fix.