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/user-templates/README.md

@@ -0,0 +1,64 @@
+# User Templates
+
+This directory allows you to create custom templates that override the default Spec Workflow templates.
+
+## How to Use Custom Templates
+
+1. **Create your custom template file** in this directory with the exact same name as the default template you want to override:
+   - `requirements-template.md` - Override requirements document template
+   - `design-template.md` - Override design document template  
+   - `tasks-template.md` - Override tasks document template
+   - `product-template.md` - Override product steering template
+   - `tech-template.md` - Override tech steering template
+   - `structure-template.md` - Override structure steering template
+
+2. **Template Loading Priority**:
+   - The system first checks this `user-templates/` directory
+   - If a matching template is found here, it will be used
+   - Otherwise, the default template from `templates/` will be used
+
+## Example Custom Template
+
+To create a custom requirements template:
+
+1. Create a file named `requirements-template.md` in this directory
+2. Add your custom structure, for example:
+
+```markdown
+# Requirements Document
+
+## Executive Summary
+[Your custom section]
+
+## Business Requirements
+[Your custom structure]
+
+## Technical Requirements
+[Your custom fields]
+
+## Custom Sections
+[Add any sections specific to your workflow]
+```
+
+## Template Variables
+
+Templates can include placeholders that will be replaced when documents are created:
+- `{{projectName}}` - The name of your project
+- `{{featureName}}` - The name of the feature being specified
+- `{{date}}` - The current date
+- `{{author}}` - The document author
+
+## Best Practices
+
+1. **Start from defaults**: Copy a default template from `../templates/` as a starting point
+2. **Keep structure consistent**: Maintain similar section headers for tool compatibility
+3. **Document changes**: Add comments explaining why sections were added/modified
+4. **Version control**: Track your custom templates in version control
+5. **Test thoroughly**: Ensure custom templates work with the spec workflow tools
+
+## Notes
+
+- Custom templates are project-specific and not included in the package distribution
+- The `templates/` directory contains the default templates which are updated with each version
+- Your custom templates in this directory are preserved during updates
+- If a custom template has errors, the system will fall back to the default template