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/ui/screens.ts

@@ -5,7 +5,7 @@ import type {
 	Progress,
 } from "../types";
 import { companions } from "../companion/characters";
-import { saveProgress } from "../storage/db";
+import { saveProgress, getProgress } from "../storage/db";
 import {
 	createExerciseState,
 	handleKeyPress,
@@ -88,6 +88,22 @@ export function initLessonScreen(
 	const backBtn = document.getElementById("lesson-back-btn") as HTMLButtonElement;
 	const state = createExerciseState(level);
 
+	// Show companion avatar (per ASST-04)
+	getProgress(_db).then((progress) => {
+		if (!progress) return;
+		const companion = companions.find(
+			(c) => c.type === progress.selectedCharacter,
+		);
+		if (!companion) return;
+		const area = document.getElementById("lesson-companion")!;
+		const avatar = document.getElementById(
+			"lesson-companion-avatar",
+		) as HTMLImageElement;
+		avatar.src = companion.avatarUrl;
+		avatar.alt = companion.name;
+		area.style.display = "flex";
+	});
+
 	// Render keyboard
 	renderKeyboard(keyboardContainer, layout, level);