Vendor Errgo and remove promise-utils

[saviro-orin]

Summary

• vendor andreogle/errgo src/index.ts byte-for-byte at commit aaa1d5153a270cde4aa808369bd486ddbe263a38
• migrate application startup, i18n initialization, client page loading, browser locale synchronization, accessibility auditing, and SSR rendering to Errgo's [error, value] tuples
• await SSR locale changes before rendering instead of racing them in the background
• remove @api3/promise-utils from package.json, the lockfile, and the installed dependency graph
• add zero-dependency Node contract tests and run them through mix lint
• update the template's async-error-handling guidance and provenance

Verification

• mise exec -- npm run test:unit --prefix assets — 2 passed
• mise exec -- npm run lint --prefix assets — 69 files clean
• mise exec -- npm run typecheck --prefix assets — app and E2E TypeScript passed
• mise exec -- mix precommit — 222 tests/doctests passed; Credo, i18n, routes, docs passed
• mise exec -- mix assets.build — browser and SSR bundles built
• vendored source SHA-256 matches upstream copy: f051197fc22e1c6a3f45d2ab05f70f39d00b6fa79fac31a698c678da768da5d9
• npm ls @api3/promise-utils --depth=0 — empty

Scope

CommonJS build-tool process-boundary catches remain intentionally unchanged because importing the TypeScript utility there would require a runtime transpiler or duplicate implementation. Application TypeScript raw catches are confined to the vendored Errgo implementation.