Add Playwright E2E harness for the RDM audit workspace

Why: U4 acceptance requires real-browser runs of the trace/audit
workspace; the repo had only a manual .playwright.ts stub gated on
PLAYWRIGHT_BASE_URL with no config, deps, or service wiring.

Evidence: config/deps/spec staged; full run driven by Dev-side
scripts/e2e.sh (seed-only smoke in progress in this task branch).

Impact: dev-only additions (@playwright/test devDep, preview proxy
segment, e2e/rdm-audit.playwright.ts); no production bundle changes.

Refs: Dev task feature/rdm-e2e-acceptance; U4 in
modules/documents/docs/proposals/2026-10-11-rdm-two-trees-trace-audit-integration.md

.gitignore

@@ -6,3 +6,6 @@ dist/
 
 # mimosa 扫描钩子的本地缓存（scripts/hooks 写入，不入库）
 .mimosa
+# Playwright 运行工件（e2e/ 用例输出；详工件在 Dev .tmp/e2e/<ts>/frontend/）
+test-results/
+playwright-report/