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/package.json

@@ -10,14 +10,14 @@
     "check": "astro check"
   },
   "dependencies": {
-    "@astrojs/cloudflare": "^9.0.0",
-    "@astrojs/tailwind": "^5.0.0",
-    "astro": "^4.0.0",
+    "@astrojs/cloudflare": "^12.6.9",
+    "@tailwindcss/vite": "^4.1.14",
+    "astro": "^5.14.1",
     "better-auth": "^1.3.13"
   },
   "devDependencies": {
     "autoprefixer": "^10.4.0",
-    "tailwindcss": "^3.4.0",
+    "tailwindcss": "^4.1.14",
     "typescript": "^5.4.0"
   }
 }