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/content-assets.mjs

@@ -0,0 +1 @@
+export default new Map();

apps/frontend/.astro/content-modules.mjs

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+export default new Map();

apps/frontend/.astro/data-store.json

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+[["Map",1,2],"meta::meta",["Map",3,4,5,6],"astro-version","5.14.1","astro-config-digest","{\"root\":{},\"srcDir\":{},\"publicDir\":{},\"outDir\":{},\"cacheDir\":{},\"compressHTML\":true,\"base\":\"/\",\"trailingSlash\":\"ignore\",\"output\":\"server\",\"scopedStyleStrategy\":\"attribute\",\"build\":{\"format\":\"directory\",\"client\":{},\"server\":{},\"assets\":\"_astro\",\"serverEntry\":\"index.js\",\"redirects\":false,\"inlineStylesheets\":\"auto\",\"concurrency\":1},\"server\":{\"open\":false,\"host\":false,\"port\":4321,\"streaming\":true,\"allowedHosts\":[]},\"redirects\":{},\"image\":{\"endpoint\":{\"route\":\"/_image\"},\"service\":{\"entrypoint\":\"astro/assets/services/sharp\",\"config\":{}},\"domains\":[],\"remotePatterns\":[],\"responsiveStyles\":false},\"devToolbar\":{\"enabled\":true},\"markdown\":{\"syntaxHighlight\":{\"type\":\"shiki\",\"excludeLangs\":[\"math\"]},\"shikiConfig\":{\"langs\":[],\"langAlias\":{},\"theme\":\"github-dark\",\"themes\":{},\"wrap\":false,\"transformers\":[]},\"remarkPlugins\":[],\"rehypePlugins\":[],\"remarkRehype\":{},\"gfm\":true,\"smartypants\":true},\"security\":{\"checkOrigin\":true},\"env\":{\"schema\":{},\"validateSecrets\":false},\"experimental\":{\"clientPrerender\":false,\"contentIntellisense\":false,\"headingIdCompat\":false,\"preserveScriptOrder\":false,\"liveContentCollections\":false,\"csp\":false,\"staticImportMetaEnv\":false,\"chromeDevtoolsWorkspace\":false,\"failOnPrerenderConflict\":false},\"legacy\":{\"collections\":false},\"session\":{\"driver\":\"cloudflare-kv-binding\",\"options\":{\"binding\":\"SESSION\"}}}"]