Refactor: split monolithic index.php into src/, views/, and README

[Copilot]

The entire application lived in a single 700-line index.php with no separation of concerns — config, DB bootstrapping, request handling, and HTML rendering all interleaved.

New structure

pastebin/
├── index.php            # Entry point: session start + 4 requires (~23 lines)
├── src/
│   ├── config.php       # DB credentials + all app constants
│   ├── db.php           # PDO bootstrap, DB/table auto-creation
│   ├── helpers.php      # Pure functions, $languages map, $basePath
│   └── handlers.php     # All POST/GET handlers; sets view variables
├── views/
│   ├── layout.php       # HTML shell, global JS
│   ├── paste.php        # Code block, comments, delete form + paste JS
│   ├── create.php       # Create form
│   └── sidebar.php      # Recent pastes
└── README.md

Key points

• Zero functional changes — identical behavior, SQL, cookie/session logic, and rendered HTML
• Each file has a single responsibility; variables flow via PHP's include scope ($paste, $comments, $recentPastes, $basePath, $languages set before views are included)
• src/config.php is the single place to touch for deployment configuration
• README.md covers directory layout, all configurable constants, installation steps, usage, and a production checklist

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