Enable R plot viewer in VS Code / Codespaces (httpgd & session watcher) #134

Problem

When using the devcontainer in VS Code / GitHub Codespaces (e.g. ghcr.io/geocompx/minimal), running R plot commands like plot(1:9) does not display any plot in the VS Code editor webview.

Root Cause

1. .devcontainer/devcontainer.json is missing r.sessionWatcher: true and r.plot.useHttpgd: true in customizations.vscode.settings.
2. The httpgd interactive graphic device R package is not installed in the base minimal container image.

Proposed Fix

1. Include httpgd in minimal/Dockerfile package installation (pak::pak()).
2. Configure VS Code R settings in .devcontainer/devcontainer.json for automatic session watching and interactive plot rendering.