feat: introduce spec-workflow and upgrade frontend dependencies

This commit introduces the spec-workflow, a set of templates for creating design, product, requirements, and other specification documents. These templates are added to both the root directory and the `apps/frontend` workspace.

Additionally, this commit upgrades the frontend development dependencies for the Astro application. The key changes include:
- Migration from `@astrojs/tailwind` to `@tailwindcss/vite` for Tailwind CSS integration.
- Deletion of the `postcss.config.cjs` file, as it is no longer needed with the new setup.
- Updates to `astro.config.mjs` to use the new Vite plugin for Tailwind CSS.
- General upgrade of Astro and other related dependencies in `apps/frontend/package.json` and the corresponding `pnpm-lock.yaml` file.

.spec-workflow/templates/product-template.md

@@ -0,0 +1,51 @@
+# Product Overview
+
+## Product Purpose
+[Describe the core purpose of this product/project. What problem does it solve?]
+
+## Target Users
+[Who are the primary users of this product? What are their needs and pain points?]
+
+## Key Features
+[List the main features that deliver value to users]
+
+1. **Feature 1**: [Description]
+2. **Feature 2**: [Description]
+3. **Feature 3**: [Description]
+
+## Business Objectives
+[What are the business goals this product aims to achieve?]
+
+- [Objective 1]
+- [Objective 2]
+- [Objective 3]
+
+## Success Metrics
+[How will we measure the success of this product?]
+
+- [Metric 1]: [Target]
+- [Metric 2]: [Target]
+- [Metric 3]: [Target]
+
+## Product Principles
+[Core principles that guide product decisions]
+
+1. **[Principle 1]**: [Explanation]
+2. **[Principle 2]**: [Explanation]
+3. **[Principle 3]**: [Explanation]
+
+## Monitoring & Visibility (if applicable)
+[How do users track progress and monitor the system?]
+
+- **Dashboard Type**: [e.g., Web-based, CLI, Desktop app]
+- **Real-time Updates**: [e.g., WebSocket, polling, push notifications]
+- **Key Metrics Displayed**: [What information is most important to surface]
+- **Sharing Capabilities**: [e.g., read-only links, exports, reports]
+
+## Future Vision
+[Where do we see this product evolving in the future?]
+
+### Potential Enhancements
+- **Remote Access**: [e.g., Tunnel features for sharing dashboards with stakeholders]
+- **Analytics**: [e.g., Historical trends, performance metrics]
+- **Collaboration**: [e.g., Multi-user support, commenting]