feat(m1): project foundation setup - Next.js 15, Tailwind, tooling, CI

- Configure Next.js 15 (App Router, Node.js runtime, serverExternalPackages for better-sqlite3/bcrypt)

- Add Tailwind CSS, PostCSS, tsconfig with @/* alias

- Create app structure (layout, page, globals.css)

- Create directory structure (lib, repositories, services, components, tests)

- Configure Vitest (alias, coverage), Playwright

- Add dependencies: next, react, better-sqlite3, bcrypt, react-markdown, etc.

- Add .env.example, CI workflow, next-env.d.ts

- Remove boilerplate src/example files

tailwind.config.ts

@@ -0,0 +1,14 @@
+import type { Config } from 'tailwindcss';
+
+const config: Config = {
+  content: [
+    './app/**/*.{js,ts,jsx,tsx,mdx}',
+    './components/**/*.{js,ts,jsx,tsx,mdx}',
+  ],
+  theme: {
+    extend: {},
+  },
+  plugins: [],
+};
+
+export default config;