fix(e2e): build packages before Playwright starts dev servers

[russellwheatley]

Summary

• Moves package builds out of Playwright's globalSetup and into the test:e2e:* npm scripts (pnpm build:packages && ...), so Vite/Angular dev servers never start against a partially-rebuilt dist/.
• globalSetup now only asserts the built package artifacts exist (assertPackagesBuilt()), instead of also being responsible for triggering the build.
• Restores the execSync import in e2e/global-setup.ts that was accidentally dropped when the now-removed runBuildPackages() helper was deleted. That ReferenceError was being swallowed and rethrown as a misleading "Auth emulator cannot start" error, failing the ui-e2e CI job before any tests could run.
• Updates AD-4 in developer-docs/decisions.md, playwright-e2e-smoke.md, and LOCAL_DEVELOPMENT.md to reflect the new build timing and Node version guidance.

Split out of #1355 so the e2e/build-infra changes can be reviewed independently of the onUpgradeFailure auth feature (AD-10).

[gemini-code-assist]

Code Review

This pull request refactors the E2E testing setup by moving the package build step out of Playwright's globalSetup and directly into the individual test:e2e:* scripts in package.json. This ensures packages are fully built before Playwright starts the dev servers, preventing issues with partially rebuilt files. The reviewer suggests optimizing these scripts by leveraging pnpm's dependency filtering (e.g., pnpm --filter=... build) to build only the necessary dependencies for each specific test, which would significantly speed up local test execution.

[mikehardy]

works for me - all the code looks ✅ and if this timing is better DX that's fine of course - this stuff is all very new so I personally have no preferences other than "make it work" :-)