Fix Chess960 FEN parsing for chess.js compatibility

- Add convertChess960Fen() function to convert Chess960 file-based
  castling notation (e.g., "HAha") to standard notation ("KQkq")
- chess.js doesn't support Chess960 FEN natively, so we convert before
  loading
- Apply conversion in all places where FEN is loaded: waiting state,
  playing state, and opponent move handler

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/lib/components/ChessGameOverlay.svelte

@@ -13,6 +13,39 @@
     let legalMoves = [];
     let moveHistory = [];
 
+    // Convert Chess960 FEN to standard FEN for chess.js compatibility
+    // Chess960 uses file-based castling (e.g., "HAha") instead of "KQkq"
+    function convertChess960Fen(fen) {
+        if (!fen) return fen;
+        const parts = fen.split(' ');
+        if (parts.length < 3) return fen;
+
+        // Check if castling field contains Chess960 notation (letters other than KQkq-)
+        const castling = parts[2];
+        if (/^[KQkq-]+$/.test(castling)) {
+            return fen; // Already standard notation
+        }
+
+        // Convert Chess960 castling to standard: any uppercase = KQ rights, any lowercase = kq rights
+        let newCastling = '';
+        if (/[A-H]/.test(castling)) {
+            // Has white castling rights - simplified to KQ
+            const whiteFiles = castling.match(/[A-H]/g) || [];
+            if (whiteFiles.length >= 2) newCastling += 'KQ';
+            else if (whiteFiles.length === 1) newCastling += 'K';
+        }
+        if (/[a-h]/.test(castling)) {
+            // Has black castling rights - simplified to kq
+            const blackFiles = castling.match(/[a-h]/g) || [];
+            if (blackFiles.length >= 2) newCastling += 'kq';
+            else if (blackFiles.length === 1) newCastling += 'k';
+        }
+        if (!newCastling) newCastling = '-';
+
+        parts[2] = newCastling;
+        return parts.join(' ');
+    }
+
     // Match state from overlay store
     let myColor = null;
 
@@ -73,8 +106,8 @@
         try {
             const chessModule = await import('chess.js');
             Chess = chessModule.Chess;
-            // Don't initialize with default position - we'll load Chess960 FEN later
+            // Don't initialize game yet - we'll create with Chess960 FEN when received
-            game = new Chess();
+            game = null;
 
             // Set up match event handler
             console.log('[ChessOverlay] Registering match event handler...');
@@ -126,9 +159,9 @@
         if (payload.status === 'waiting') {
             myColor = payload.yourColor;
 
-            // Load the Chess960 starting position
+            // Create Chess960 game with the starting position
-            if (game && payload.fen) {
+            if (Chess && payload.fen) {
-                game.load(payload.fen, { chess960: true });
+                game = new Chess(convertChess960Fen(payload.fen));
                 updateBoardDisplay();
             }
 
@@ -143,8 +176,9 @@
             const session = nakama.getSession();
             myColor = payload.whiteId === session?.user_id ? 'w' : 'b';
 
-            if (game) {
+            // Create Chess960 game with current position
-                game.load(payload.fen, { chess960: true });
+            if (Chess && payload.fen) {
+                game = new Chess(convertChess960Fen(payload.fen));
             }
 
             gamesOverlay.setMode('playing');
@@ -163,8 +197,9 @@
         } else if (payload.status === 'spectating') {
             myColor = null; // Spectator has no color
 
-            if (game) {
+            // Create Chess960 game with current position
-                game.load(payload.fen, { chess960: true });
+            if (Chess && payload.fen) {
+                game = new Chess(convertChess960Fen(payload.fen));
             }
 
             gamesOverlay.setMode('spectating');
@@ -184,7 +219,7 @@
 
     function handleOpponentMove(payload) {
         if (game) {
-            game.load(payload.fen, { chess960: true });
+            game.load(convertChess960Fen(payload.fen));
         }
         moveHistory = [...moveHistory, payload.move];