fix(01): add lesson cancel button and handle level 6 completion

- Back button in lesson screen to return to forest without completing
- "Alle Stufen geschafft!" message when all 6 levels are done
- Cap currentLevel at MAX_LEVEL to prevent out-of-bounds progression

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

src/styles/main.css

@@ -259,6 +259,15 @@ h3 {
 	transform: scale(1.05);
 }
 
+.forest__continue-btn--done {
+	background: var(--accent-gold);
+	cursor: default;
+}
+
+.forest__continue-btn--done:hover {
+	transform: none;
+}
+
 /* Lesson screen */
 .lesson {
 	display: flex;
@@ -269,6 +278,29 @@ h3 {
 	padding: 2rem;
 	min-height: 100vh;
 	background: var(--bg-cream);
+	position: relative;
+}
+
+.lesson__back-btn {
+	position: absolute;
+	top: 1rem;
+	left: 1rem;
+	font-family: "Quicksand", sans-serif;
+	font-size: 1rem;
+	font-weight: 600;
+	color: var(--text-warm);
+	background: var(--bg-forest);
+	border: 2px solid var(--accent-green);
+	border-radius: 12px;
+	padding: 0.5rem 1rem;
+	cursor: pointer;
+	transition: background 0.2s ease, transform 0.1s ease;
+}
+
+.lesson__back-btn:hover {
+	background: var(--accent-green);
+	color: white;
+	transform: scale(1.05);
 }
 
 .lesson__letter-area {