feat(ui): Docker support, code quality tooling, and landing page foundation

[Thommyy7]

Summary

Covers four issues across two concerns — developer experience tooling
and the landing page foundation. These are grouped together as they
are all non-blocking setup work that can land in a single clean PR.

Changes

UI #9 — Docker support

• packages/ui/Dockerfile — multi-stage build: deps → builder → runner
  using Next.js standalone output
• packages/ui/.dockerignore — excludes node_modules, .next, all .env files

UI #10 — ESLint, Prettier, TypeScript strict mode

• .prettierrc — semi, singleQuote, trailingComma all, printWidth 100
• .prettierignore — excludes .next, out, node_modules
• eslint.config.mjs — extends next/core-web-vitals, next/typescript,
  prettier (prevents ESLint/Prettier rule conflicts)
• tsconfig.json — strict: true, noUncheckedIndexedAccess: true enabled
• package.json — format and format:check scripts already present

UI #11 — Landing page foundation

• src/components/landing/Navbar.tsx — logo left, GitHub + Open App
  buttons right, transparent background over hero
• src/app/page.tsx — landing page root at /, assembles all sections
  with dark background and subtle grid overlay

UI #12 — Hero section and AnimatedDemo

• src/components/landing/HeroSection.tsx — headline, subheadline,
  CTA buttons, overflow-hidden on section to prevent mobile overflow
• src/components/landing/AnimatedDemo.tsx — self-contained demo
  with no real API calls, animates: idle → typing → loading → result,
  tab switch triggers exit/enter animation sequence

How to test

# Code quality
npm run lint
npm run format:check

# Landing page
npm run dev
# Visit http://localhost:3000 — landing page with hero and animated demo
# Visit http://localhost:3000/app — search page unchanged

# Docker
docker build -t stellar-explain-ui .
# Or from monorepo root:
docker compose up --build

Closes

Closes #157
Closes #158
Closes #159
Closes #160

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