ci(e2e): run Playwright from the prebuilt image instead of downloading browsers (refs #73)

The verify-e2e lane downloaded ~150 MB of Chromium (npx playwright install) on
every verify-stack run. Use the official mcr.microsoft.com/playwright image with
browsers pre-baked; npm install still pins @playwright/test from tests/e2e, and
the image tag is kept in lockstep with that version. Verified the exact
create + docker cp + start flow launches the baked browser with no download.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/frontend-decisions.md

@@ -85,12 +85,14 @@ with the submit form (S-08c, #67); any deviation from NL DS will be recorded her
 - **Runtime config.** The app fetches `/config.json` before bootstrap (`main.ts`); `appConfig` is a
   factory. The dev default (`public/config.json`) points at `localhost:8180`; the Docker image bakes
   the compose value (`keycloak:8080`). One build, per-environment OIDC authority.
-- **e2e runs inside the compose network.** `infra/run-e2e-check.sh` runs Playwright in a `node`
-  container on `cg`, so the browser reaches Keycloak as `keycloak:8080` — the **same issuer** the BFF
-  validates against (resolves the browser-vs-container mismatch, ADR-0010). Chromium is installed at
-  runtime, so there's no Playwright-image-version pinning to keep in sync. The spec is copied in
-  (`docker cp`), not mounted, so it leaves nothing root-owned on the host. Wired as `verify-e2e` in
-  the `verify-stack` CI job.
+- **e2e runs inside the compose network.** `infra/run-e2e-check.sh` runs Playwright in a container on
+  `cg`, so the browser reaches Keycloak as `keycloak:8080` — the **same issuer** the BFF validates
+  against (resolves the browser-vs-container mismatch, ADR-0010). It uses the official
+  `mcr.microsoft.com/playwright:<version>` image with browsers pre-baked, rather than downloading
+  ~150 MB of Chromium on every run (issue #73) — the image tag is kept in lockstep with
+  `tests/e2e/package.json`'s `@playwright/test` version. The spec is copied in (`docker cp`), not
+  mounted, so it leaves nothing root-owned on the host. Wired as `verify-e2e` in the `verify-stack`
+  CI job.
 - **e2e treats the portal origin as secure.** In-network the portal is served over plain HTTP on a
   non-localhost origin (`http://self-service`), which is **not a secure context**, so Web Crypto
   (`crypto.subtle`) is unavailable. angular-auth-oidc-client needs it for the PKCE code challenge, so