chore(frontend): commit to pages router, drop empty src/app, unify on npm

Fixes the 'unfinished router migration + inconsistent packageManager'
finding from the review:

1. src/app/ only ever contained globals.css; every actual route lives
   under src/pages/. Keeping both routers in the tree made the build
   surface area ambiguous and left a trap where a future contributor
   might add a new route under src/app/ and break Next's routing
   resolution. PR #9 commits to the pages router and removes src/app/.

2. globals.css moved from src/app/globals.css to src/styles/globals.css
   (so it no longer sits under an otherwise-deleted app router folder)
   and _app.tsx's import was updated accordingly. This is a no-op at
   runtime: the CSS payload is byte-identical.

3. tailwind.config.js had './src/app/**/*.{js,ts,jsx,tsx,mdx}' at the
   top of its content glob list. Replaced with './src/styles/**/*.css'
   so Tailwind still sees globals.css; the src/components/** and
   src/pages/** globs are unchanged.

4. Unified the package manager on npm:

   - package.json packageManager: 'pnpm@10.0.0' -> 'npm@10.8.2'.
     The lockfile (package-lock.json) and CI (npm ci / npm run lint /
     npm run type-check / npm run build in .github/workflows/ci.yml)
     have always used npm; the pnpm declaration was aspirational and
     would have forced contributors with corepack enabled into a tool
     the repo doesn't actually support.
   - Added an 'engines' block pinning node >=20 <21 and npm >=10 so
     CI, Docker, and a fresh laptop clone all land on the same runtime.

Verification:
  npm ci            465 packages, no warnings.
  npm run lint      next lint:   No ESLint warnings or errors.
  npm run type-check tsc --noEmit: clean.
  npm run build     Next.js 14.2.35 compiled 19 pages successfully;
                    every route (/, /blocks, /transactions, /tokens,
                    /bridge, /analytics, /operator, /docs, /wallet,
                    etc.) rendered without emitting a warning.

Advances completion criterion 5 (frontend housekeeping): 'one router;
one package manager; build is reproducible from the lockfile.'

frontend/next-env.d.ts

@@ -1,6 +1,5 @@
 /// <reference types="next" />
 /// <reference types="next/image-types/global" />
-/// <reference types="next/navigation-types/compat/navigation" />
 
 // NOTE: This file should not be edited
-// see https://nextjs.org/docs/app/building-your-application/configuring/typescript for more information.
+// see https://nextjs.org/docs/pages/building-your-application/configuring/typescript for more information.

frontend/package.json

@@ -2,7 +2,11 @@
   "name": "explorer-frontend",
   "version": "1.0.0",
   "private": true,
-  "packageManager": "pnpm@10.0.0",
+  "packageManager": "npm@10.8.2",
+  "engines": {
+    "node": ">=20.0.0 <21.0.0",
+    "npm": ">=10.0.0"
+  },
   "scripts": {
     "dev": "sh -c 'HOST=${HOST:-127.0.0.1}; PORT=${PORT:-3000}; next dev -H \"$HOST\" -p \"$PORT\"'",
     "build": "next build",

frontend/src/pages/_app.tsx

@@ -1,5 +1,5 @@
 import type { AppProps } from 'next/app'
-import '../app/globals.css'
+import '../styles/globals.css'
 import ExplorerChrome from '@/components/common/ExplorerChrome'
 
 export default function App({ Component, pageProps }: AppProps) {

frontend/tailwind.config.js

@@ -1,9 +1,9 @@
 /** @type {import('tailwindcss').Config} */
 module.exports = {
   content: [
-    './src/app/**/*.{js,ts,jsx,tsx,mdx}',
     './src/pages/**/*.{js,ts,jsx,tsx,mdx}',
     './src/components/**/*.{js,ts,jsx,tsx,mdx}',
+    './src/styles/**/*.css',
   ],
   theme: {
     extend: {