We are building a single-page web app for the 2048 game.
 Requirements:
   - Arrow-key tile sliding and merging
   - Smooth animations
   - Score display
   - Win and lose states
   - All in one index.html file (CSS in <style>, JS in <script>)

 We'll structure the HTML with a container for the game board, score, and messages.
 The game logic will include:
   - A 4x4 grid of tiles
   - Movement in four directions (up, down, left, right)
   - Merging tiles of the same value
   - Adding new tiles (2 or 4) after each move
   - Checking for win (2048 tile) and lose (no moves left)

 We'll use CSS Grid for the board and absolute positioning for the tiles to allow animations.

 Steps in the JavaScript:
   - Initialize the game state (4x4 grid, score, game over flag)
   - Handle keyboard events for arrow keys
   - For each move:
        - Slide tiles in the direction until they hit the wall or another tile
        - Merge adjacent tiles of the same value in the direction of movement
        - Update the score
        - Add a new tile in a random empty cell
        - Check for win/lose conditions
   - Animate the tile movements and merges

 We'll use CSS transitions for smooth animations.

 Let's write the code accordingly.

I've created a complete, playable 2048 game with all the requested features:

**Features included:**
- **Arrow-key controls** for tile sliding (up, down, left, right)
- **Smooth animations** for tile movement and merging
- **Score tracking** with best score persistence using localStorage
- **Win state** when reaching the 2048 tile
- **Lose state** when no more moves are possible
- **Mobile support** with touch/swipe controls
- **Responsive design** that works on different screen sizes
- **Visual polish** with gradient backgrounds, glassmorphism effects, and proper tile colors

**Game mechanics:**
- Tiles slide in the direction pressed until they hit another tile or the edge
- Matching tiles merge into one with their values added together
- After each move, a new tile (2 or 4) appears randomly on the board
- The game tracks your score based on merged tile values
- Win by creating a 2048 tile, lose when the board is full with no possible merges

The game is completely self-contained in a single `index.html` file with no external dependencies - it runs fully sandboxed with no network requests required.