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/astro.config.mjs

@@ -1,12 +1,16 @@
-import { defineConfig } from 'astro/config';
-import cloudflare from '@astrojs/cloudflare';
-import tailwind from '@astrojs/tailwind';
+// @ts-check
+import { defineConfig } from "astro/config";
+import cloudflare from "@astrojs/cloudflare";
+import tailwindcss from "@tailwindcss/vite";
 
+// https://astro.build/config
 export default defineConfig({
-  output: 'server',
+  output: "server",
   adapter: cloudflare(),
-  integrations: [tailwind()],
+  vite: {
+    plugins: [tailwindcss()],
+  },
   typescript: {
-    strict: true
-  }
+    strict: true,
+  },
 });