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/package-lock.json

@@ -20,6 +20,7 @@
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@@ -3010,6 +3027,53 @@
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