feat(01-04): wire lesson screen UI with keyboard and falling letters

- Lesson screen HTML with letter-area and keyboard containers
- initLessonScreen: renders keyboard, shows letters, handles keypress
- Correct key dissolves letter with animation, advances to next
- Wrong key does nothing (per D-03), correct key stays highlighted
- startLesson in app.ts: manages lesson lifecycle and cleanup
- onComplete: updates progress, unlocks next level, saves to IndexedDB
- CSS: floating letter animation, dissolve effect on correct press

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

src/app.ts

@@ -1,8 +1,9 @@
-import type { ScreenName } from "./types";
-import { openDB, getProgress } from "./storage/db";
-import { initWelcomeScreen } from "./ui/screens";
+import type { ScreenName, KeyboardLayout } from "./types";
+import { openDB, getProgress, saveProgress } from "./storage/db";
+import { initWelcomeScreen, initLessonScreen } from "./ui/screens";
 
 let db: IDBDatabase;
+let lessonCleanup: (() => void) | null = null;
 
 export function showScreen(name: ScreenName): void {
 	document.querySelectorAll(".screen").forEach((el) => {
@@ -18,14 +19,57 @@ export function getDB(): IDBDatabase {
 	return db;
 }
 
+export async function startLesson(
+	level: number,
+	layout: KeyboardLayout,
+): Promise<void> {
+	// Clean up previous lesson if any
+	if (lessonCleanup) {
+		lessonCleanup();
+		lessonCleanup = null;
+	}
+
+	showScreen("lesson");
+
+	lessonCleanup = initLessonScreen(db, level, layout, async (completedLevel: number) => {
+		// Update progress after lesson completion
+		const progress = await getProgress(db);
+		if (!progress) return;
+
+		// Add level to completedLevels if not already there
+		if (!progress.completedLevels.includes(completedLevel)) {
+			progress.completedLevels.push(completedLevel);
+		}
+
+		// Advance to next level if this was the current level (LEVL-02, LEVL-03)
+		if (completedLevel === progress.currentLevel) {
+			progress.currentLevel = completedLevel + 1;
+		}
+
+		// Increment session count
+		progress.totalSessions++;
+
+		// Add today's date if not already recorded
+		const today = new Date().toISOString().split("T")[0]!;
+		if (!progress.sessionDates.includes(today)) {
+			progress.sessionDates.push(today);
+		}
+
+		await saveProgress(db, progress);
+
+		// Navigate to forest (per D-07: skip reward in Phase 1)
+		showScreen("forest");
+	});
+}
+
 export async function initApp(): Promise<void> {
 	db = await openDB();
 	const progress = await getProgress(db);
 	if (progress && progress.selectedCharacter) {
-		// Returning user — skip welcome, go to forest
+		// Returning user -- go to forest
 		showScreen("forest");
 	} else {
-		// First time — show welcome
+		// First time -- show welcome
 		initWelcomeScreen(db, showScreen);
 		showScreen("welcome");
 	}