chore: Refactor chess logic to MVC pattern and enhance board representation

modules/api/src/main/scala/de/nowchess/api/board/Board.scala

@@ -0,0 +1,31 @@
+package de.nowchess.api.board
+
+opaque type Board = Map[Square, Piece]
+
+object Board:
+
+  def apply(pieces: Map[Square, Piece]): Board = pieces
+
+  extension (b: Board)
+    def pieceAt(sq: Square): Option[Piece] = b.get(sq)
+    def withMove(from: Square, to: Square): (Board, Option[Piece]) =
+      val captured = b.get(to)
+      val updated  = b.removed(from).updated(to, b(from))
+      (updated, captured)
+    def pieces: Map[Square, Piece] = b
+
+  val initial: Board =
+    val backRank: Vector[PieceType] = Vector(
+      PieceType.Rook, PieceType.Knight, PieceType.Bishop, PieceType.Queen,
+      PieceType.King, PieceType.Bishop, PieceType.Knight, PieceType.Rook
+    )
+    val entries = for
+      fileIdx <- 0 until 8
+      (color, rank, pieceType) <- Seq(
+        (Color.White, Rank.R1, backRank(fileIdx)),
+        (Color.White, Rank.R2, PieceType.Pawn),
+        (Color.Black, Rank.R8, backRank(fileIdx)),
+        (Color.Black, Rank.R7, PieceType.Pawn)
+      )
+    yield Square(File.values(fileIdx), rank) -> Piece(color, pieceType)
+    Board(entries.toMap)

modules/api/src/main/scala/de/nowchess/api/board/Color.scala

@@ -6,3 +6,7 @@ enum Color:
   def opposite: Color = this match
     case White => Black
     case Black => White
+
+  def label: String = this match
+    case White => "White"
+    case Black => "Black"

modules/api/src/main/scala/de/nowchess/api/board/PieceType.scala

@@ -2,3 +2,11 @@ package de.nowchess.api.board
 
 enum PieceType:
   case Pawn, Knight, Bishop, Rook, Queen, King
+
+  def label: String = this match
+    case Pawn   => "Pawn"
+    case Knight => "Knight"
+    case Bishop => "Bishop"
+    case Rook   => "Rook"
+    case Queen  => "Queen"
+    case King   => "King"