Wire leave-group successor picker UI test into automated suite

Original task: make the ad-hoc browser test for the leave-group
successor chooser dialog a persistent automated test that runs
alongside the existing api-server backend tests.

Changes:
- The Playwright spec at
  `artifacts/teaboy-os/tests/leave-group-successor.spec.mjs` was
  already present (covers the cancel path and the
  pick-specific-successor path, seeds its own users + group via
  Postgres, cleans up in afterAll). Verified it passes against the
  running web + api workflows.
- Installed @playwright/test + pg as devDependencies in
  @workspace/teaboy-os (already in package.json, ran pnpm install
  to materialize) and downloaded the Playwright Chromium browser.
- Added a root `test` script in package.json that runs the
  api-server tests and then the teaboy-os e2e tests (sequential
  with `&&`) so a single command exercises both layers.
- Registered a single `test` validation command that runs api +
  e2e sequentially. Initially registered them as two separate
  commands but a parallel validation run caused api-server session
  flakes (401s), so collapsed into one sequential command.
- Updated `scripts/post-merge.sh` to also install the Playwright
  Chromium browser after every merge, and bumped the post-merge
  timeout to 120s to accommodate the (cached) browser install.

Validation: combined sequential `test` validation passes — all 15
api-server tests pass and both new e2e tests pass.

Note: `artifacts/teaboy-os/public/opengraph.jpg` was modified by
another process (the workflow build) and is not part of this
task's intended changes.

Replit-Task-Id: ab9b6d6f-b68b-47ea-94e5-c34352afeb64

package.json

@@ -6,7 +6,8 @@
     "preinstall": "sh -c 'rm -f package-lock.json yarn.lock; case \"$npm_config_user_agent\" in pnpm/*) ;; *) echo \"Use pnpm instead\" >&2; exit 1 ;; esac'",
     "build": "pnpm run typecheck && pnpm -r --if-present run build",
     "typecheck:libs": "tsc --build",
-    "typecheck": "pnpm run typecheck:libs && pnpm -r --filter \"./artifacts/**\" --filter \"./scripts\" --if-present run typecheck"
+    "typecheck": "pnpm run typecheck:libs && pnpm -r --filter \"./artifacts/**\" --filter \"./scripts\" --if-present run typecheck",
+    "test": "pnpm --filter @workspace/api-server test && pnpm --filter @workspace/teaboy-os test:e2e"
   },
   "private": true,
   "devDependencies": {