Fix production-build template E2E environment - #2

Summary

• provide the app-template Playwright server with explicit mock-only Vite settings
• keep ordinary production-build environment validation unchanged
• make the template E2E suite independent of untracked developer .env files

Validation

• pnpm --filter @stack/template-app lint
• pnpm --filter @stack/template-app typecheck
• pnpm --filter @stack/template-app test (7 passed)
• pnpm --filter @stack/template-app build
• pnpm exec prettier --check templates/app/playwright.config.ts
• Playwright production server build contains the configured mock URL and app name; local browser execution is blocked by missing host Chromium libraries (libnspr4.so and related system dependencies). Exact-head GitHub CI installs these dependencies and exercises all 18 tests.

Unblocks the exact-main baseline required before ADR-013 / WP-08A console migration.