feat(02-05): add companion greeting to forest and avatar to lesson screen

- Add forest-greeting companion area in HTML with avatar + text
- Add lesson-companion area in HTML above letter area
- Wire getGreeting() call for returning users in app.ts
- Show companion avatar in lesson screen via getProgress lookup
- Fix pre-existing noUncheckedIndexedAccess errors in fallbacks.ts (Rule 3)

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

src/app.ts

@@ -2,6 +2,8 @@ import type { ScreenName } from "./types";
 import { openDB, getProgress, saveProgress } from "./storage/db";
 import { initWelcomeScreen, initLessonScreen } from "./ui/screens";
 import { initForestScreen } from "./forest/scene";
+import { getGreeting } from "./companion/companion";
+import { companions } from "./companion/characters";
 
 let db: IDBDatabase;
 let lessonCleanup: (() => void) | null = null;
@@ -89,6 +91,27 @@ export async function initApp(): Promise<void> {
 		// Returning user -- init forest and go there
 		await initForestScreen(db, startLessonFromForest);
 		showScreen("forest");
+
+		// Show companion greeting (per COMP-01, D-14)
+		const companion = companions.find(
+			(c) => c.type === progress.selectedCharacter,
+		);
+		if (companion) {
+			const greetingEl = document.getElementById("forest-greeting")!;
+			const avatarEl = document.getElementById(
+				"forest-greeting-avatar",
+			) as HTMLImageElement;
+			const textEl = document.getElementById("forest-greeting-text")!;
+
+			avatarEl.src = companion.avatarUrl;
+			avatarEl.alt = companion.name;
+			greetingEl.style.display = "flex";
+
+			// Fetch greeting async (shows immediately with avatar, text fills in)
+			getGreeting(progress.selectedCharacter, db).then((text) => {
+				textEl.textContent = text;
+			});
+		}
 	} else {
 		// First time -- show welcome, then forest after setup
 		initWelcomeScreen(db, async (screen: ScreenName) => {