test: add Playwright webServer block so E2E never targets a stale dev server

[ericfitz]

Summary

The Playwright config (playwright.config.ts) has no webServer block; E2E tests blindly target http://localhost:4200 and use whatever process happens to be listening. On 2026-08-01/02 this produced a materially misleading result: a full DFD E2E run reported 42/58 passing while being served by a stale, long-lived ng serve process that predated a dependency change (#826) which had actually broken every fresh dev-server boot on main (see #762's override collision; diagnosed during #446 execution). The passing run masked a hard bootstrap crash for over a day.

Acceptance Criteria

• playwright.config.ts gains a webServer entry (e.g. command: pnpm dev:e2e / ng serve --configuration=e2e --port 4200, url: http://localhost:4200) so Playwright owns dev-server lifecycle.
• reuseExistingServer: false locally (or gated so CI always starts fresh), guaranteeing E2E can never run against a pre-existing server process.
• Backend prerequisite (localhost:8080) documented or health-checked in global setup with a clear failure message.
• e2e/README.md updated to reflect that manual ng serve is no longer required.

Notes

• Motivating incident: during the @antv/x6 v3 upgrade (deps: @antv/x6 major upgrade v2 -> v3 #446), E2E failures were misattributed to the upgrade branch; root cause was main broken by chore(deps): bump dependencies #826 + the stale @angular/build>vite override (build: drop @angular/build>vite pnpm override once Angular dev-server supports Rolldown-based Vite 8 #762/build: eliminate ng serve startup deprecation warnings #763), invisible because the E2E run reused a pre-chore(deps): bump dependencies #826 server.
• Consider --trace on-first-retry as a default while touching the config.