…ustom

When `dashboards-prebuilt-insights-dashboards` is enabled, the secondary
navigation now shows three tabs:

- All Dashboards (`?filter=all`): combined view of custom + prebuilt
- Custom Dashboards: the existing no-filter view, renamed from "All
  Dashboards"
- Sentry Built (`?filter=onlyPrebuilt`): unchanged

The primary sidebar still lands on Custom Dashboards (the no-filter URL)
to preserve existing behavior; the new All tab is opt-in via the
secondary nav.

DAIN-1637

…f prebuilt

The vast majority of orgs have the prebuilt dashboards feature enabled,
so the per-org label split adds noise without helping the rare
non-prebuilt case.

…b enum

Replace the ad-hoc isOnlyPrebuilt/isAllDashboards booleans with a single
DashboardsTab state derived from the URL filter, plus mapping tables for
the page title and API query params. The mutually exclusive boolean
states made the filter logic harder to follow and easier to break with a
new tab.

Add an "All Dashboards" entry that links to ?filter=all when prebuilt
dashboards are enabled, and rename the no-filter entry to "Custom
Dashboards" so it matches the secondary nav after the recent split.

…ards"

… off

When `dashboards-prebuilt-insights-dashboards` is disabled, there's no
sibling "Sentry Built" tab to contrast against, so "Custom Dashboards"
loses its meaning. Fall back to "All Dashboards" for that case in the
page title, secondary nav link, and command palette entry. Tests now
enable the prebuilt flag so they exercise the rename branch.

…shboards-tab-to-custom-dashboards