Add link to experiments running in GrowthBook on list/catalog view

What?

During the interim transition period (and after when GrowthBook will be the only experiment source), the TKUI catalog renders a link to experiments configured in GrowthBook.

• New GET /growthbook-experiments-summary endpoint returns { enabled, count, url } whether GrowthBook is an enabled source (include_gb=true), how many GB experiments are currently running, and the environment-specific GrowthBook UI URL (staging TKUI links to staging GB, production TKUI links to production GB).
• countGrowthBookExperiments() counts only running GB-source rows in experiments_cache determined by the cached date window rather than GrowthBook's isActive flag which can go stale when an experiment leaves the GB API.
• ui_url added to the growthbook config block in config.dev/prod.yaml.
• List.vue renders the link whenever GrowthBook is enabled and a URL is configured. The summary fetch runs in parallel with the registered- experiments fetch and upon failure, the link remains hidden without blocking the list.

Why?

Because we are no longer including read-only GB-sourced experiments in the TK frontend, we are instead providing a link from the TKUI frontend to the GB UI for viewing GB-originated experiments.

Notes:

• The rendering of the GrowthBook experiments link is gated behind include_gb=true so while we have yet to include GB as a source, this MR should be safe to merge on production. It will render the link if GB is included as a source and if there are any actively running experiments in GB.
• The new config variables should be included in the Kubernetes helm charts and values added accordingly per staging and production environments.

Testing Instructions

• Make sure include_gb=true in config.local.yaml
• View the new link header on the list/catalog view:

• Test that the link doesn't show up if include_gb=false in config.local.yaml
• Verify that the new link header to GB UI is not visible and the normal list/catalog view renders

Bug: T426464