Add dashboard screenshot harness to the dev tooling (#840)

* Add dashboard screenshot harness to the dev tooling

dashboard-dev-screenshots.py drives headless Chromium (Playwright)
against the dashboard dev stack and captures how Kibana, OpenSearch
Dashboards, Grafana, and Splunk actually render the current sample
data — for end-to-end verification of dashboard changes and PR
evidence. It encodes the platform quirks that otherwise cost time to
rediscover: fixed render waits instead of networkidle (Kibana/OSD
dashboards poll forever), pinning OSD to the global tenant (a stale
private-tenant copy silently screenshots old dashboards), and driving
the Grafana and Splunk login forms rather than HTTP basic auth. The
output directory is gitignored; usage is documented in
dashboards/README.md.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* Address Copilot review findings on the screenshot harness

- Only require OPENSEARCH_INITIAL_ADMIN_PASSWORD / SPLUNK_PASSWORD when
  the osd / splunk targets are selected, and fail fast before launching
  Playwright with a clear message naming the missing variable(s).
- Honor GRAFANA_USER (defaulting to admin) to match the dev bootstrap
  script instead of hard-coding the Grafana username.
- Print a full traceback to stderr when a target fails, instead of just
  the exception message, while still continuing to the next target.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* Always close the browser when a screenshot target fails

Copilot round 2: each target only closed its Chromium instance on the
happy path, so a mid-run Playwright failure leaked a headless browser
while the script continued to the next target. Wrap each target body in
try/finally; closing the browser also closes its contexts and pages, so
the single b.close() covers the OSD context too.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* Exit with an install hint when playwright is missing

Copilot round 3: playwright is deliberately not a project dependency, so
a bare import failure produced a raw ModuleNotFoundError traceback.
Catch ImportError and exit with the same one-time install command the
module docstring documents.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

.gitignore

@@ -151,3 +151,4 @@ parsedmarc/resources/maps/unknown_domains.txt
 parsedmarc/resources/maps/domain_info.tsv
 coverage.json
 junit.xml
+dashboard-screenshots/