style: improve pill layout for mobile

- Wrap lesson label in span for mobile hiding
- Truncate module name with ellipsis if too long
- Hide "Lesson/Lektion" label on mobile, show only "1 / 3"
- Add lessonLabel i18n keys for EN/DE

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/unit/renderer.test.js

@@ -129,10 +129,10 @@ describe("Renderer Module", () => {
 			const element = document.getElementById("level-indicator");
 
 			renderLevelIndicator(element, 3, 10);
-			expect(element.textContent).toBe("Lesson 3 of 10");
+			expect(element.textContent).toBe("Lesson 3 / 10");
 
 			renderLevelIndicator(element, 1, 5);
-			expect(element.textContent).toBe("Lesson 1 of 5");
+			expect(element.textContent).toBe("Lesson 1 / 5");
 		});
 	});