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/ui/screens.ts

@@ -30,7 +30,9 @@ export function initWelcomeScreen(
 		card.className = "companion-card";
 		card.dataset.companion = c.type;
 		card.innerHTML = `
-			<span class="companion-card__emoji">${c.emoji}</span>
+			<img class="companion-card__avatar" src="${c.avatarUrl}" alt="${c.name}"
+				onerror="this.style.display='none'; this.nextElementSibling.style.display='block';">
+			<span class="companion-card__emoji" style="display:none">${c.emoji}</span>
 			<span class="companion-card__name">${c.name}</span>
 		`;
 		card.addEventListener("click", () => {