feat(frontend): Next.js 15 + React 19 + Tailwind 4 + lightweight-charts scaffold

[obchain]

Summary

Frontend skeleton lands. First TypeScript surface in the repo. Establishes the Next.js + Tailwind + lightweight-charts stack the landing (#26) and chart (#27) issues build on.

Stack

Layer	Pin
Framework	Next.js 15.1.6 (app router)
React	19.0.0
Styling	Tailwind 4.0.17 (via @tailwindcss/postcss)
Charts	lightweight-charts 4.2.2 (available, not yet rendered — #27)
Package mgr	pnpm 9.15.0
Node	≥ 22 (dev box on 22.22.0)
Type checker	TypeScript 5.7.3 strict
Linter	ESLint 9 flat config bridging next/core-web-vitals + next/typescript via FlatCompat
Formatter	Prettier 3.4.2 — 100 col, double quotes, trailing comma

Routes

App router file layout:

frontend/app/
├── layout.tsx              # root + dark default body
├── globals.css             # @import "tailwindcss" + theme vars
├── page.tsx                # /          (placeholder landing)
├── api/page.tsx            # /api       (placeholder API page)
├── chart/[index]/page.tsx  # /chart/bvol or /chart/evol (placeholder)
└── methodology/page.tsx    # /methodology (placeholder)

Five user-facing routes plus _not-found. Every body is a thin placeholder — copy + chart UI land in #26 / #27. Scaffold ships first so those issues don't have to do scaffold work + their own work in one PR.

Verification

Step	Result
pnpm install	Clean, 332 deps, 22.7 s
pnpm typecheck (tsc --noEmit)	Zero errors
pnpm lint (next lint)	"No ESLint warnings or errors"
pnpm build	5 routes built, all 146 B + 105 kB shared First Load
pnpm dev smoke	curl :3000/ → 200, 29152 bytes, Tailwind classes render, <title>VolX</title>

First-attempt build error (resolved)

@tailwindcss/postcss@4.0.0 is incompatible with the postcss version Next 15.1 ships — build failed with Missing field 'negated' on ScannerOptions.sources. Bumped both tailwindcss and @tailwindcss/postcss to 4.0.17; build clean. Noted for future Tailwind 4 + Next 15 work.

Out of scope

• Landing page copy + design (Frontend: landing page (live BVOL/EVOL + sparkline) #26)
• /chart/[index] candle rendering + WS subscribe (Frontend: /chart/[index] candle chart + live WS #27)
• /methodology body (M0 Draft METHODOLOGY.md #6 content goes here later)
• /api body (M3+ if API keys revive)
• Test framework (Vitest / Playwright — separate later issue if needed)
• Storybook / component dev environment
• Vercel deploy config (local-only direction)

Notes

• Tailwind 4 uses CSS-first config (@theme inline in globals.css) — no tailwind.config.ts. This is the v4 way.
• next.config.ts has experimental.typedRoutes enabled so future <Link href="/chart/bvol"> gets compile-time path validation.
• pnpm lockfile committed (pnpm-lock.yaml). node_modules excluded via existing .gitignore.
• next-env.d.ts regenerated by next lint; matches the upstream skeleton with the /api-reference/config/typescript doc link form.

Test plan

• pnpm install clean
• pnpm typecheck clean
• pnpm lint clean
• pnpm build clean
• pnpm dev serves :3000 with HTTP 200
• Visited /, /methodology, /api, /chart/bvol, /chart/evol — all render placeholder

Fixes #25