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/index.html

@@ -135,6 +135,18 @@
           <!-- ④ 最低分样本逐条复核 -->
           <div class="section-label">④ 最低分样本（点击展开逐条复核）</div>
           <div class="lowest-table" id="lowest-table"></div>
+
+          <!-- ⑤ 优化建议（optimization_advisor: true 时显示） -->
+          <div id="advice-section" hidden>
+            <div class="section-label">⑤ 优化建议 OPTIMIZATION ADVICE</div>
+            <div class="panel advice-panel">
+              <div class="advice-header">
+                <span class="advice-badge">AI 诊断报告</span>
+                <span class="advice-model" id="advice-model-label"></span>
+              </div>
+              <div class="advice-body" id="advice-body"></div>
+            </div>
+          </div>
         </div>
       </section>