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.

apps/frontend/.spec-workflow/config.example.toml

@@ -0,0 +1,72 @@
+# Spec Workflow MCP Server Configuration File
+# ============================================
+#
+# This is an example configuration file for the Spec Workflow MCP Server.
+# Copy this file to 'config.toml' in the same directory to use it.
+#
+# Configuration Precedence:
+# 1. Command-line arguments (highest priority)
+# 2. Config file settings
+# 3. Built-in defaults (lowest priority)
+#
+# All settings are optional. Uncomment and modify as needed.
+# Please note that not all MCP clients will support loading this config file due to the nature of where they are running from.
+
+# Project directory path
+# The root directory of your project where spec files are located.
+# Note: You may have to use double slashes (\\) instead of single slashes (/) on Windows or for certain clients.
+# Supports tilde (~) expansion for home directory.
+# Default: current working directory
+# projectDir = "."
+# projectDir = "~/my-project"
+# projectDir = "/absolute/path/to/project"
+
+# Dashboard port
+# The port number for the web dashboard.
+# Must be between 1024 and 65535.
+# Default: ephemeral port (automatically assigned)
+# port = 3000
+
+# Auto-start dashboard
+# Automatically launch the dashboard when the MCP server starts.
+# The dashboard will open in your default browser.
+# Default: false
+# autoStartDashboard = false
+
+# Dashboard-only mode
+# Run only the web dashboard without the MCP server.
+# Useful for standalone dashboard usage.
+# Default: false
+# dashboardOnly = false
+
+# Language
+# Set the interface language for internationalization (i18n).
+# Available languages depend on your installation.
+# Common values: "en" (English), "ja" (Japanese), etc.
+# Default: system language or "en"
+# lang = "en"
+
+# Example configurations:
+# =====================
+
+# Example 1: Development setup with auto-started dashboard
+# ----------------------------------------------------------
+# projectDir = "~/dev/my-project"
+# autoStartDashboard = true
+# port = 3456
+
+# Example 2: Production MCP server without dashboard
+# ---------------------------------------------------
+# projectDir = "/var/projects/production"
+# autoStartDashboard = false
+
+# Example 3: Dashboard-only mode for viewing specs
+# -------------------------------------------------
+# projectDir = "."
+# dashboardOnly = true
+# port = 8080
+
+# Example 4: Japanese language interface
+# ---------------------------------------
+# lang = "ja"
+# autoStartDashboard = true