Add React webview system with pnpm workspaces

[EhabY]

Summary

Adds a React-based webview system to the extension using pnpm workspaces. This provides the foundation for building rich UI panels (like the Tasks panel) with React while maintaining type safety between the extension and webviews.

Architecture

packages/
├── shared/                      # Shared types and React utilities
│   ├── src/
│   │   ├── index.ts             # WebviewMessage type
│   │   └── react/               # VS Code webview hooks
│   │       ├── api.ts           # postMessage, getState, setState
│   │       └── hooks.ts         # useMessage, useVsCodeState
│   ├── extension.d.ts           # Types exposed to extension (excludes react/)
│   └── vite.config.base.ts      # Shared Vite config factory
└── tasks/                       # Tasks panel webview (placeholder)
    └── src/
        ├── index.tsx            # Entry point
        └── App.tsx              # Root component

src/webviews/
├── util.ts                      # getWebviewHtml() with CSP
└── tasks/
    └── TasksPanel.ts            # WebviewViewProvider

Key Features

• Type-safe message passing: WebviewMessage type shared between extension and webviews
• React hooks: useMessage() for receiving messages, useVsCodeState() for persisted state
• Secure CSP: Nonce-based Content Security Policy for script execution
• Type isolation: Extension can only import types from @coder/shared (not React code) via extension.d.ts
• Proper lifecycle management: Disposable pattern for event listeners, visibility change handling

Changes

• Add pnpm workspace configuration
• Add @coder/shared package with types and React hooks
• Add @coder/tasks-webview package (placeholder Tasks panel)
• Add src/webviews/util.ts with getWebviewHtml() helper
• Add src/webviews/tasks/TasksPanel.ts WebviewViewProvider
• Register Tasks panel in package.json (visible when authenticated + devMode)
• Add ESLint rules for React
• Add webviews documentation to CONTRIBUTING.md

[code-asher]

Nice, looks like a good base for webviews.