Enable reliable Codespaces access link via forwarded Vite port and clear usage docs

[Copilot]

The issue was to make app access via a Codespace link straightforward and explicit. This PR adds a minimal Codespaces runtime config and documents exactly how to open/share the running app URL from the Ports panel.

• Codespaces runtime config

  • Added .devcontainer/devcontainer.json to standardize startup and port behavior in Codespaces:
    • Node 22 devcontainer image
    • postCreateCommand with dependency install
    • Forwarded app port 3000 (Vite)
    • Port metadata for discoverability (label) and non-intrusive auto-forward behavior (notify)

• Access-link documentation

  • Updated README.md with a dedicated GitHub Codespaces access section:
    • run npm run dev
    • open port 3000 via Ports → Open in Browser
    • clarify default private visibility and how to make the link public (Public) for unauthenticated sharing

• Config snippet

  {
    "postCreateCommand": "npm install",
    "forwardPorts": [3000],
    "portsAttributes": {
      "3000": {
        "label": "Vite dev server",
        "onAutoForward": "notify"
      }
    }
  }

• Screenshot
  

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