Establish deterministic frontend checks

[fellnerse]

Summary

• standardize the repository on pnpm 8.15.8 and remove the stale Yarn lockfile
• add explicit build, generate, test, lint, and formatting scripts
• make CI installs reproducible and checks non-mutating
• update the GitHub Actions used by CI and hosting deployments
• verify generated pages and required PWA assets after static generation
• prevent ESLint and Prettier from inspecting generated output

Notes

• Node remains at 16 in GitHub Actions deliberately; the Node and final Nuxt 2 upgrade will be a separate migration step
• six existing Vue files were normalized to make the formatting check pass
• Firebase Functions are unchanged

Verification

• pnpm install --frozen-lockfile
• pnpm format:check
• pnpm lint
• pnpm test
• pnpm ci-dev-build
• pnpm test:generated
• git diff --check

[github-actions]

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