feat(03-03): add SVG forest background and 4x3 grid HTML/CSS structure

- SVG woodland background with sky gradient, clouds, sun, trees, hills, ground
- 4x3 CSS grid overlay for forest element placement
- Fade-in + scale-up animation keyframes for new elements

src/styles/main.css

@@ -378,6 +378,69 @@ h3 {
 	line-height: 1.4;
 }
 
+/* Forest Scene */
+.forest__scene {
+	position: relative;
+	width: 100%;
+	max-width: 800px;
+	margin: 0 auto 1.5rem;
+	border-radius: 16px;
+	overflow: hidden;
+	box-shadow: 0 4px 12px rgba(0, 0, 0, 0.08);
+}
+
+.forest__bg {
+	display: block;
+	width: 100%;
+	height: auto;
+}
+
+.forest__grid {
+	position: absolute;
+	top: 0;
+	left: 0;
+	width: 100%;
+	height: 100%;
+	display: grid;
+	grid-template-columns: repeat(4, 1fr);
+	grid-template-rows: repeat(3, 1fr);
+	padding: 1rem;
+	gap: 0.5rem;
+}
+
+.forest__element {
+	display: flex;
+	align-items: center;
+	justify-content: center;
+}
+
+.forest__element img {
+	max-width: 100%;
+	max-height: 100%;
+	object-fit: contain;
+	border-radius: 8px;
+	filter: drop-shadow(0 2px 4px rgba(0, 0, 0, 0.15));
+}
+
+.forest__element--new {
+	animation: forest-element-appear 1.2s ease-out forwards;
+}
+
+@keyframes forest-element-appear {
+	0% {
+		opacity: 0;
+		transform: scale(0.3);
+	}
+	60% {
+		opacity: 1;
+		transform: scale(1.05);
+	}
+	100% {
+		opacity: 1;
+		transform: scale(1);
+	}
+}
+
 /* Keyboard */
 .keyboard {
 	display: flex;