fix(config-ui): keep grafana deep links working with an external grafana

[DoDiODev]

Summary

Fixes two bugs with the Grafana links of the config-ui. Part of #9219
(section B). Defaults and proxy mode are unchanged.

External Grafana redirect keeps path and query

With USE_EXTERNAL_GRAFANA=true, nginx.conf answered every /grafana/...
request with 302 ${GRAFANA_ENDPOINT} and dropped the path and the query. All
dashboard links (e.g. /grafana/d/<uid>/<slug>?orgId=1) therefore ended on the
Grafana home page.

• The redirect now appends the part after /grafana from $request_uri
  (still URL-encoded, query string included), via a map in nginx.conf.
• nginx.sh drops a trailing slash from GRAFANA_ENDPOINT, so
  https://grafana.example.com/ does not produce //.
• Proxy mode (default) is unchanged.

"Check Dashboard" button in the onboarding

The button opened import.meta.env.DEVLAKE_DASHBOARD_URL_*. These are
build-time variables; the released image is built without them, so the button
opened an empty tab. The example values in config-ui/env.example also point
to the MySQL dashboard UIDs, while the PostgreSQL dashboards use a -pg
suffix.

• New getDashboardURL(plugin): uses the configured URL if set, otherwise the
  Grafana dashboard search for the plugin (e.g.
  /grafana/dashboards?query=GitHub, respecting DEVLAKE_GRAFANA_PATH). The
  titles are the same for MySQL and PostgreSQL, so this works for both.
• Unit tests for the helper.

Docs

• config-ui/README.md: GRAFANA_ENDPOINT / USE_EXTERNAL_GRAFANA in the
  Docker image.
• config-ui/env.example: the dashboard URLs are optional, and the
  PostgreSQL UIDs use the -pg suffix.

Does this close any open issues?

Part of #9219 (section B).

Screenshots

N/A

Other Information

Tested with the image built from this branch and from main (ee0079ab3):

Scenario	main	this PR
External: /grafana/d/KXWvOFQnz-pg/github?orgId=1&var-repo_id=All	❌ redirects to https://grafana.example.com/	✅ https://grafana.example.com/d/KXWvOFQnz-pg/github?orgId=1&var-repo_id=All
External: encoded query (var-repo=a%20b&var-team=%2Fdev)	❌ dropped	✅ kept as is
External with a sub path (GRAFANA_ENDPOINT=https://example.com/grafana)	❌ https://example.com/grafana	✅ https://example.com/grafana/d/…
External: /grafana/	✅	✅
Proxy mode: deep link, /grafana/, SPA route	✅	✅ unchanged
Built bundle	no dashboard URL (empty tab)	falls back to the dashboard search

The search URL was checked against Grafana 13.2.3 in a headless browser:
/dashboards?query=GitHub lists only the matching dashboard.
tsc, eslint, vitest and vite build pass. Full CI mirror on my fork:
run