refactor: update devcontainer and Biome configuration (#27)

- Remove ESLint and Prettier in favor of Biome
- Configure Biome as default formatter for TS/JS/JSON/MD
- Add development helper extensions
- Update VSCode settings for Biome integration

Closes #27

.github/copilot-instructions.md

@@ -10,6 +10,28 @@ This project is a web user interface (UI) for a CRUD (Create, Read, Update, Dele
 *   **Language:** TypeScript, JavaScript (client-side scripts), HTML, CSS
 *   **Styling:** Plain CSS (`src/styles/global.css`)
 *   **Data:** Using Astro's built-in API routes in `src/pages/api/` with a temporary in-memory store (`src/data/store.ts`). **The goal is to eventually replace the in-memory store with a persistent database.**
+*   **Development Environment:** VS Code Dev Container using private Docker image (`ghcr.io/acedanger/finance-devcontainer:latest`)
+
+## Development Environment
+
+*   **Dev Container:** The project uses a VS Code Dev Container for consistent development environments.
+    *   Container configuration in `.devcontainer/devcontainer.json`
+    *   Uses a private container image hosted on GitHub Container Registry
+    *   Image: `ghcr.io/acedanger/finance-devcontainer:latest`
+    *   Includes all necessary development tools and extensions
+    *   Configured with GitHub CLI and authentication
+    *   Features Docker-in-Docker support for additional container needs
+*   **Container Features:**
+    *   Node.js and npm pre-installed
+    *   Git and GitHub CLI configured
+    *   TypeScript support
+    *   VS Code extensions pre-configured
+    *   Docker-in-Docker capability
+    *   Automatic GitHub authentication
+*   **Authentication:**
+    *   Requires GitHub Personal Access Token for private container access
+    *   Token should be configured in `.devcontainer/.env`
+    *   GitHub CLI authentication handled in post-start command
 
 ## Current State & Key Features
 
@@ -36,6 +58,10 @@ This project is a web user interface (UI) for a CRUD (Create, Read, Update, Dele
 
 ## File Structure Overview
 
+*   `.devcontainer/`: Development container configuration
+    *   `devcontainer.json`: VS Code Dev Container configuration
+    *   `Dockerfile`: Base container definition (for reference)
+    *   `.env.example`: Template for container environment variables
 *   `src/components/`: Reusable UI components.
 *   `src/data/`: Data store and persistence layer.
 *   `src/layouts/`: Base page layout(s).