feat: Add devcontainer configuration

[jasonspriggs]

This PR adds a .devcontainer directory to provide a standardized local development environment using VS Code Remote - Containers.

Key features include:

• Base image mcr.microsoft.com/devcontainers/javascript-node:20 for Node.js 20.
• Docker-in-Docker feature to run bun run docker:up for the local Postgres database.
• Forwarding ports 3000 (App), 3002 (Portal), and 5432 (Postgres).
• Pre-configured VS Code extensions (Prettier, Tailwind, Prisma, Bun).
• A setup.sh post-create command that installs bun, automatically copies .env.example to .env for apps/app, apps/portal, and packages/db, and installs all project dependencies.

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PR created automatically by Jules for task 2243194844622812248 started by @jasonspriggs

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