feat: Add CSS styles for CanvasArea component

frontend/src/components/CanvasArea.css

@@ -0,0 +1,26 @@
+.canvas-area {
+  flex: 1;
+  background-color: var(--background);
+  overflow: hidden;
+  position: relative;
+}
+
+.canvas-placeholder {
+  position: absolute;
+  top: 50%;
+  left: 50%;
+  transform: translate(-50%, -50%);
+  text-align: center;
+  color: var(--secondary);
+  font-family: var(--font-family);
+}
+
+.canvas-placeholder p {
+  margin: 0;
+  padding: 0;
+}
+
+.placeholder-subtext {
+  font-size: var(--font-size-sm);
+  margin-top: var(--spacing-sm);
+}