fix(rithmomachia): change undock icon to pop-out arrow

Replace confusing fullscreen-like icon (⛶) with pop-out arrow (⤴️)
for the undock button. The new icon better conveys the action of
"pop out to floating mode" rather than looking like fullscreen/maximize.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

apps/web/src/arcade-games/rithmomachia/components/PlayingGuideModal.tsx

@@ -487,7 +487,7 @@ export function PlayingGuideModal({
               onMouseLeave={(e) => (e.currentTarget.style.background = '#e5e7eb')}
               title="Undock guide (return to floating mode)"
             >
-              ⛶
+              ⤴️
             </button>
           )}
           {/* Bust-out button (only if not already standalone/docked and not very narrow) */}