normalise FENs - trim and fuill move to 1 on addition

lib/chesstrainer/fen.ex

@@ -7,6 +7,39 @@ defmodule Chesstrainer.FEN do
     |> color_initial_to_color_atom()
   end
 
+  @doc """
+  Normalizes a FEN string for storage.
+
+  Trims surrounding whitespace and rewrites a fullmove number of `"0"` to
+  `"1"`. The fullmove rewrite is a chess.js-compatibility fix — chess.js
+  rejects fullmove `0` as invalid even though many board editors default
+  to it. Other FEN fields are left untouched; structural errors are
+  surfaced by the Endgame changeset, not silently masked here.
+
+  ## Examples
+
+      iex> Chesstrainer.FEN.normalize("  8/8/3k4/8/8/3K1R2/8/8 w - - 0 0\\n  ")
+      "8/8/3k4/8/8/3K1R2/8/8 w - - 0 1"
+
+      iex> Chesstrainer.FEN.normalize("8/8/3k4/8/8/3K1R2/8/8 w - - 0 5")
+      "8/8/3k4/8/8/3K1R2/8/8 w - - 0 5"
+
+      iex> Chesstrainer.FEN.normalize("not a fen")
+      "not a fen"
+  """
+  @spec normalize(String.t()) :: String.t()
+  def normalize(fen) when is_binary(fen) do
+    trimmed = String.trim(fen)
+
+    case String.split(trimmed, " ") do
+      [board, active, castling, en_passant, halfmove, "0"] ->
+        Enum.join([board, active, castling, en_passant, halfmove, "1"], " ")
+
+      _ ->
+        trimmed
+    end
+  end
+
   defp color_initial_to_color_atom("b"), do: :black
   defp color_initial_to_color_atom("w"), do: :white
   defp color_initial_to_color_atom(_), do: nil