feat(webapp): add optimization advice section to report UI

- index.html: add section ⑤ advice block (hidden by default, shown when advice_markdown present)
- report.js: add renderAdvice() called in render(), simple Markdown→HTML converter
- app.js: add noise_sensitivity / factual_correctness / semantic_similarity to shortMetric map
- app.css: add .advice-panel, .advice-badge, .advice-md styles (purple left-border theme)

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

webapp/static/js/app.js

@@ -119,6 +119,9 @@ const App = {
       answer_relevancy: "ans.rel.",
       context_recall: "ctx.recall",
       context_precision: "ctx.prec.",
+      noise_sensitivity: "noise.sens.",
+      factual_correctness: "fact.corr.",
+      semantic_similarity: "sem.sim.",
     };
     return map[name] || name;
   },