Add paused-state vignette border to game world view

Draw a black gradient border (50% alpha at the viewport edges fading to
transparent toward the center) while the game is paused (speed 0x), making
the paused state hard to miss.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Y7N59FsLA5e2kuVdqe4Uhc

src/ui/GameWorldView.cpp

@@ -431,6 +431,7 @@ void GameWorldView::paintGL()
     drawBeams(painter);
     drawOverlays(painter);
     drawScreenSpace(painter);
+    drawPauseBorder(painter);
     drawReplayOverlay(painter);
 }
 
@@ -1695,6 +1696,46 @@ void GameWorldView::drawScreenSpace(QPainter& /*painter*/)
 {
 }
 
+void GameWorldView::drawPauseBorder(QPainter& painter)
+{
+    if (m_gameSpeedMultiplier > 0.0) { return; }
+
+    // Border thickness in pixels, capped so it never exceeds half the viewport on
+    // very small windows (which would leave no un-darkened center).
+    const qreal borderPx = std::min<qreal>(
+        100.0, std::min(width(), height()) / 2.0);
+    if (borderPx <= 0.0) { return; }
+
+    const QColor edge(0, 0, 0, 128);        // 50% black at the viewport edge
+    const QColor inner(0, 0, 0, 0);         // fully transparent toward the center
+
+    painter.save();
+
+    // Each side is a linear gradient fading from `edge` at the viewport border to
+    // `inner` toward the center. Corners overlap and read as a natural vignette.
+    QLinearGradient left(0, 0, borderPx, 0);
+    left.setColorAt(0.0, edge);
+    left.setColorAt(1.0, inner);
+    painter.fillRect(QRectF(0, 0, borderPx, height()), left);
+
+    QLinearGradient right(width(), 0, width() - borderPx, 0);
+    right.setColorAt(0.0, edge);
+    right.setColorAt(1.0, inner);
+    painter.fillRect(QRectF(width() - borderPx, 0, borderPx, height()), right);
+
+    QLinearGradient top(0, 0, 0, borderPx);
+    top.setColorAt(0.0, edge);
+    top.setColorAt(1.0, inner);
+    painter.fillRect(QRectF(0, 0, width(), borderPx), top);
+
+    QLinearGradient bottom(0, height(), 0, height() - borderPx);
+    bottom.setColorAt(0.0, edge);
+    bottom.setColorAt(1.0, inner);
+    painter.fillRect(QRectF(0, height() - borderPx, width(), borderPx), bottom);
+
+    painter.restore();
+}
+
 void GameWorldView::drawReplayOverlay(QPainter& painter)
 {
     if (!m_replayPlayer) { return; }

src/ui/GameWorldView.h

@@ -131,6 +131,10 @@ private:
     void drawBeams(QPainter& painter);
     void drawOverlays(QPainter& painter);
     void drawScreenSpace(QPainter& painter);
+    // Vignette-style border shown while the game is paused (speed 0x) to make the
+    // paused state hard to miss: a black frame whose alpha fades from 50% at the
+    // viewport edges to 0% toward the center.
+    void drawPauseBorder(QPainter& painter);
     void drawReplayOverlay(QPainter& painter);
 
     float    getTilePx() const;