Rules:
• You may only click the bottom-most remaining letter of each column.
• Each letter you click is appended to the word row at the bottom.
• On an N×N board, only N-letter words are accepted (3×3 → 3 letters, 4×4 → 4 letters, 5×5 → 5 letters, etc.).
• Words are checked against the dictionary (VALID_WORDS, uppercase).
• Selected tiles appear empty in the grid and show up below, chained to form the word.
• Accepted words remove tiles permanently. Clear all tiles to win.