Add last-move arrow + destination pulse so opponent moves are obvious

Draws a lichess-style arrow from the last move's origin to its destination
(persists until you reply) and pulses the destination square when a new move
lands, so the opponent's move can't be missed.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

client/app.js

@@ -171,7 +171,10 @@ class BoardUI {
   setOrientation(o) { this.orient = o; this.render(); }
   load(fen, lastMove) {
     try { this.chess.load(fen); } catch {}
+    const prev = this.lastMove;
     this.lastMove = lastMove || null;
+    // flag a genuinely new move so render() can pulse the destination square
+    this._pulse = !!(this.lastMove && (!prev || prev.from !== this.lastMove.from || prev.to !== this.lastMove.to));
     this.selected = null;
     this.render();
   }
@@ -195,6 +198,7 @@ class BoardUI {
       const dark = (FILES.indexOf(file) + rank) % 2 === 0;
       const cell = el(`<div class="sq ${dark ? 'dark' : 'light'}" data-sq="${sq}"></div>`);
       if (this.lastMove && (sq === this.lastMove.from || sq === this.lastMove.to)) cell.classList.add('last');
+      if (this.lastMove && sq === this.lastMove.to) { cell.classList.add('lastto'); if (this._pulse) cell.classList.add('pulse'); }
       if (sq === this.selected) cell.classList.add('sel');
       if (sq === kingSq) cell.classList.add('check');
 
@@ -211,8 +215,41 @@ class BoardUI {
       if (legalTo.has(sq)) cell.appendChild(el(`<div class="hint ${p ? 'cap' : ''}"></div>`));
       board.appendChild(cell);
     });
+    this._drawLastArrow();
+    this._pulse = false;
     if (this.pendingPromo) this._renderPromo();
   }
+  // column/row (0..7) of a square in the CURRENT board orientation
+  _cellPos(sq) {
+    const f = FILES.indexOf(sq[0]);
+    const r = +sq[1];
+    return this.orient === 'w'
+      ? { col: f, row: 8 - r }
+      : { col: 7 - f, row: r - 1 };
+  }
+  // draw a lichess-style arrow from the last move's origin to its destination
+  _drawLastArrow() {
+    const m = this.lastMove;
+    if (!m || !m.from || !m.to) return;
+    const a = this._cellPos(m.from), b = this._cellPos(m.to);
+    const x1 = a.col + 0.5, y1 = a.row + 0.5;
+    const x2 = b.col + 0.5, y2 = b.row + 0.5;
+    const dx = x2 - x1, dy = y2 - y1;
+    const len = Math.hypot(dx, dy) || 1;
+    const ux = dx / len, uy = dy / len;
+    const head = 0.42;                       // arrowhead length (in square units)
+    const start = 0.34;                       // pull the tail out from the piece center
+    const sx = x1 + ux * start, sy = y1 + uy * start;
+    const tipX = x2 - ux * 0.06, tipY = y2 - uy * 0.06;
+    const baseX = tipX - ux * head, baseY = tipY - uy * head;
+    const px = -uy, py = ux;                   // perpendicular
+    const hw = 0.24;                           // half-width of arrowhead base
+    const svg = el(`<svg class="arrows" viewBox="0 0 8 8" preserveAspectRatio="none"></svg>`);
+    svg.innerHTML =
+      `<line x1="${sx}" y1="${sy}" x2="${baseX}" y2="${baseY}" class="arrow-line"/>` +
+      `<polygon points="${tipX},${tipY} ${baseX + px * hw},${baseY + py * hw} ${baseX - px * hw},${baseY - py * hw}" class="arrow-head"/>`;
+    this.board.appendChild(svg);
+  }
   _kingSquare(color) {
     for (const sq of this.squares()) { const p = this.chess.get(sq); if (p && p.type === 'k' && p.color === color) return sq; }
     return null;

public/style.css

@@ -116,11 +116,23 @@ main { flex: 1; width: 100%; }
 
 /* board */
 .board {
-  width: min(72vh, 100%); aspect-ratio: 1; margin: 0 auto;
+  width: min(72vh, 100%); aspect-ratio: 1; margin: 0 auto; position: relative;
   display: grid; grid-template-columns: repeat(8, 1fr); grid-template-rows: repeat(8, 1fr);
   border-radius: 10px; overflow: hidden; border: 1px solid var(--line);
   box-shadow: 0 18px 60px rgba(0,0,0,.5); touch-action: none; user-select: none;
 }
+/* last-move arrow overlay */
+.arrows { position: absolute; inset: 0; width: 100%; height: 100%; pointer-events: none; z-index: 4; }
+.arrow-line { stroke: var(--red); stroke-width: .17; stroke-linecap: round; opacity: .82; }
+.arrow-head { fill: var(--red); opacity: .82; }
+/* destination-square emphasis + one-shot pulse when a new move lands */
+.sq.lastto::before { content: ''; position: absolute; inset: 0; box-shadow: inset 0 0 0 3px var(--red); opacity: .55; z-index: 1; }
+.sq.pulse::before { animation: sqpulse 1.1s ease-out 2; }
+@keyframes sqpulse {
+  0% { box-shadow: inset 0 0 0 3px var(--red); opacity: 1; }
+  60% { box-shadow: inset 0 0 0 7px var(--red); opacity: .2; }
+  100% { box-shadow: inset 0 0 0 3px var(--red); opacity: .55; }
+}
 .sq { position: relative; display: flex; align-items: center; justify-content: center; }
 .sq.light { background: var(--light-sq); }
 .sq.dark { background: var(--dark-sq); }