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>

index.html

@@ -28,7 +28,16 @@
 			</div>
 		</section>
 		<section id="screen-forest" class="screen"></section>
-		<section id="screen-lesson" class="screen"></section>
+		<section id="screen-lesson" class="screen">
+			<div class="lesson">
+				<div class="lesson__letter-area" id="letter-area">
+					<!-- Current letter appears here -->
+				</div>
+				<div class="lesson__keyboard" id="lesson-keyboard">
+					<!-- Keyboard rendered here -->
+				</div>
+			</div>
+		</section>
 		<section id="screen-reward" class="screen"></section>
 		<section id="screen-parent" class="screen"></section>