test(dashboard): add Playwright e2e scaffolding with smoke spec

Milestone A of the frontend test plan. Sets up the test rig — config,
globalSetup that probes the backend and seeds an admin session into
storageState, lightweight fixtures, and a 3-test smoke spec — without
yet covering any user journeys (those land in Milestone B).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

dashboard/.prettierignore

@@ -2,3 +2,9 @@
 build
 node_modules
 package-lock.json
+
+# Playwright generated artifacts
+playwright-report
+test-results
+tests/e2e/.auth
+tests/e2e/.results