Docs: update CONTRIBUTING.md for Node 22+/pnpm 11+ requirements and modular namespace i18n architecture

Problem Description

In CONTRIBUTING.md, two key sections contain outdated guidance that causes friction for new contributors:

1. Environment Requirements & Local Setup:

   • Node.js: Listed as Node.js version 20+, but root package.json specifies "engines": { "node": ">=22.22.0" }. Attempting setup on Node 20 triggers engine mismatch warnings or failures.
   • Python: Listed as Python version 3.8+, but Python 3.8 is end-of-life and unsupported by modern Django; apps/api/Dockerfile.api specifies python:3.12.10-alpine.
   • Package Manager: pnpm (version 11+ via Corepack) is not listed in Requirements, despite being the required package manager for the monorepo.
   • Setup instructions: The step-by-step instructions omit pnpm install before pnpm dev, causing missing binary/module errors if contributors run steps manually or if setup.sh encounters issues.

2. Translation & i18n Contribution Guide (packages/i18n):

   • The docs describe an obsolete architecture where translations live in a single monolithic translations.json alongside a core.json.
   • In reality, translations are organized into 28 feature-based namespace files (e.g., common.json, auth.json, work-item.json, project.json) governed by NAMESPACES in packages/i18n/src/constants/namespaces.ts.
   • Step 3 instructs contributors to modify a nonexistent private importLanguageFile(language: TLanguage) method; packages/i18n/src/core/instance.ts uses resourcesToBackend to dynamically import ../locales/${language}/${namespace}.json, requiring zero import code changes when adding a new language.

Proposed Changes

• Update CONTRIBUTING.md Requirements to specify Node.js >=22.22.0, Python 3.12+, and pnpm >=11 (or corepack enable pnpm).
• Clarify manual setup steps to explicitly include pnpm install prior to running pnpm dev.
• Update the i18n architecture guide in CONTRIBUTING.md to reflect the 28 namespace JSON files, explain how keys map to namespaces, and provide the correct steps for adding a new language (updating TLanguage, SUPPORTED_LANGUAGES, and creating the namespace JSON files without needing nonexistent import methods).

Start with CONTRIBUTING.md, then verify the requirements against root package.json and apps/api/Dockerfile.api. Read packages/i18n/src/constants/namespaces.ts and packages/i18n/src/core/instance.ts to confirm the namespace files and dynamic loading guidance. Done means CONTRIBUTING.md accurately documents setup requirements, pnpm install, the namespace architecture, and the language-addition steps.