feat(02-05): add avatar images to welcome screen with emoji fallback

- Extend CompanionDefinition with avatarUrl field using Vite static imports
- Replace emoji-only companion cards with avatar img + onerror emoji fallback
- Add companion-card__avatar CSS (80px round) and companion-area styles (48px)
- Add vite-env.d.ts for PNG import type declarations

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/styles/main.css

@@ -109,6 +109,14 @@ h3 {
 	background: var(--bg-forest);
 }
 
+.companion-card__avatar {
+	width: 80px;
+	height: 80px;
+	border-radius: 50%;
+	object-fit: cover;
+	margin-bottom: 0.5rem;
+}
+
 .companion-card__emoji {
 	font-size: 3rem;
 }
@@ -344,6 +352,32 @@ h3 {
 	}
 }
 
+/* Companion area (forest greeting + lesson avatar) */
+.companion-area {
+	display: flex;
+	align-items: center;
+	gap: 0.75rem;
+	padding: 0.75rem 1rem;
+	background: var(--bg-cream);
+	border-radius: 1rem;
+	margin-bottom: 1rem;
+}
+
+.companion-area__avatar {
+	width: 48px;
+	height: 48px;
+	border-radius: 50%;
+	object-fit: cover;
+	flex-shrink: 0;
+}
+
+.companion-area__text {
+	font-family: "Nunito", sans-serif;
+	font-size: 1rem;
+	color: var(--text-warm);
+	line-height: 1.4;
+}
+
 /* Keyboard */
 .keyboard {
 	display: flex;