[Upgrade] Refactor frontend structure and setup for v2

[thxforall]

Description

This issue outlines the key tasks for upgrading and restructuring the frontend codebase of decoded-app to v2. The goal is to modernize the stack, improve maintainability, and establish a scalable, modular architecture inspired by Toss’s frontend structure.

✅ Goals
• Apply Yarn Plug’n’Play (PnP) (eliminate node_modules)
• Upgrade stack to Next.js 15, Tailwind CSS 4, TypeScript 5, and React 19
• Adopt Toss-style directory structure (src/app, src/domains, src/shared, etc.)
• Use Webpack (explicitly not using Turbopack)
• Set up config files: tsconfig.json, tailwind.config.ts, .eslintrc, .prettierrc, etc.
• Use plain .md files for documentation (not using VitePress)
• Create foundational docs and README.md
• Define Git commit & PR strategy (inspired by “Tidy First” principles)

📂 Tasks
• Create feature/decoded-v2 branch and clean up old structure
• Set up Yarn PnP and update package.json
• Create new Toss-style directory structure
• Add entry files (layout.tsx, page.tsx, globals.css)
• Add essential config files (TS, ESLint, Tailwind)
• Write version-up steps in docs/version-upgrade.md
• Prepare README.md with updated vision and tech stack
• Ensure commit discipline and structure/behavioral separation