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>

.gitignore

@@ -30,6 +30,13 @@ config.local.toml
 /dashboard/build
 /dashboard/.env
 
+# Dashboard — Playwright E2E
+/dashboard/tests/e2e/.auth
+/dashboard/tests/e2e/.results
+/dashboard/playwright-report
+/dashboard/test-results
+/dashboard/.playwright
+
 # Caddy
 /caddy/data
 /caddy/config