docs: update progress for completed subtask 2.4

.auto-claude/specs/001-conceptual-explanations/build-progress.txt

@@ -122,3 +122,49 @@ Implementation details:
 
 Committed: 0e39cff
 Status: ✓ Completed
+
+
+=== 2026-01-11 - Subtask 2.3 Completed ===
+Modified renderer.js renderLesson() function to populate the concept section.
+
+Implementation details:
+- Added logic to populate concept section elements in renderLesson() function
+- Get references to concept DOM elements by ID:
+  * concept-section (details element)
+  * concept-explanation (explanation text container)
+  * concept-diagram (optional diagram container)
+  * concept-container-vs-item (optional Flexbox/Grid distinction)
+- Conditional rendering based on lesson.concept existence:
+  * Show concept section when lesson.concept exists with explanation
+  * Hide concept section when concept is not defined
+- Field population:
+  * explanation: uses textContent (safe for user content, required field)
+  * diagram: uses innerHTML (supports SVG markup, optional field)
+  * containerVsItem: uses textContent (safe for user content, optional field)
+- Clear optional fields when not present to prevent stale data from previous lessons
+- Follows existing code patterns in renderer.js
+- Proper null checks for all DOM elements
+
+Committed: e21bca1
+Status: ✓ Completed
+
+
+=== 2026-01-11 - Subtask 2.4 Completed ===
+Added 'whyThisWorks' translation key for the concept section heading.
+
+Implementation details:
+- Added translation key to src/i18n.js for all 6 supported languages
+- Translations added:
+  * en (English): "Why This Works"
+  * de (German): "Warum das funktioniert"
+  * pl (Polish): "Dlaczego to działa"
+  * es (Spanish): "Por qué funciona"
+  * ar (Arabic): "لماذا يعمل هذا"
+  * uk (Ukrainian): "Чому це працює"
+- Translation key matches the data-i18n attribute in the concept section summary element
+- Follows existing i18n.js structure and patterns
+- Placed in "Instructions" comment section for consistency
+- Phase 2 (UI Components) is now complete - all 4 subtasks finished
+
+Committed: 3c08b45
+Status: ✓ Completed