feat: show learning progress in web app

.planning/phases/008-learning-progress-view/008-RESEARCH.md

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+# Phase 8 Research: Learning Progress View
+
+## Findings
+
+The backend already provides progress projection after answer submission. The
+frontend only needs to fetch and summarize the three endpoints after grading.
+
+Useful MVP display:
+
+- profile target role and stack
+- top concept mastery states
+- readiness percentage
+- next challenge concept, ladder level, and question
+
+## Recommendation
+
+- Refresh progress automatically after successful answer submission.
+- Add a manual refresh button for recovery.
+- Use empty state before the first answer.
+- Keep evidence labels compact so they do not overwhelm the practice surface.