feat(icons): swap drawer "Chess" menu icon to MaterialSymbols.ChessKnight

- Replace R.drawable.ic_chess (custom drawable) with MaterialSymbols.ChessKnight
  (U+F25E) in the navigation drawer's Chess menu entry. Knight is the most
  recognizable chess piece silhouette.
- Drop ic_chess.xml.

Chess board pieces (ChessPieceVectors.kt, CBurnett set) stay as ImageVector —
Material Symbols glyphs are monochrome and can't do the white-with-black-outline
style that keeps pieces visible on both light and dark squares.

https://claude.ai/code/session_016gV9gEaSud6kDMdcaqy8ti

commons/src/commonMain/kotlin/com/vitorpamplona/amethyst/commons/icons/symbols/MaterialSymbols.kt

@@ -59,6 +59,7 @@ object MaterialSymbols {
     val Chat = MaterialSymbol("\uE0C9")
     val Check = MaterialSymbol("\uE5CA")
     val CheckCircle = MaterialSymbol("\uF0BE")
+    val ChessKnight = MaterialSymbol("\uF25E")
     val Checklist = MaterialSymbol("\uE6B1")
     val ChevronRight = MaterialSymbol("\uE5CC")
     val Circle = MaterialSymbol("\uEF4A")