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

index.html

@@ -34,6 +34,30 @@
 					<img id="forest-greeting-avatar" class="companion-area__avatar" src="" alt="">
 					<p id="forest-greeting-text" class="companion-area__text"></p>
 				</div>
+				<div class="forest__scene" id="forest-scene">
+					<svg class="forest__bg" viewBox="0 0 800 400" xmlns="http://www.w3.org/2000/svg">
+						<defs>
+							<linearGradient id="sky-grad" x1="0" y1="0" x2="0" y2="1">
+								<stop offset="0%" stop-color="#E8F4FD"/>
+								<stop offset="100%" stop-color="#F0F8E8"/>
+							</linearGradient>
+						</defs>
+						<rect x="0" y="0" width="800" height="300" fill="url(#sky-grad)"/>
+						<ellipse cx="150" cy="60" rx="60" ry="25" fill="white" opacity="0.5"/>
+						<ellipse cx="550" cy="80" rx="70" ry="20" fill="white" opacity="0.4"/>
+						<circle cx="700" cy="50" r="30" fill="#F8E8A0" opacity="0.6"/>
+						<g opacity="0.3">
+							<ellipse cx="80" cy="240" rx="40" ry="60" fill="#7DB87D"/>
+							<ellipse cx="720" cy="250" rx="35" ry="55" fill="#6DA86D"/>
+							<ellipse cx="400" cy="230" rx="45" ry="70" fill="#8DC88D"/>
+						</g>
+						<ellipse cx="200" cy="320" rx="250" ry="80" fill="#A8D8A8"/>
+						<ellipse cx="650" cy="330" rx="200" ry="60" fill="#B8E0B0"/>
+						<rect x="0" y="300" width="800" height="100" fill="#C8E6C0" rx="0"/>
+					</svg>
+					<div class="forest__grid" id="forest-grid">
+					</div>
+				</div>
 				<div class="forest__level-map" id="level-map">
 					<!-- Level cards rendered dynamically -->
 				</div>