Story 27: Coverage tracking (full-stack)

Add end-to-end coverage tracking: backend collects vitest coverage,
records metrics with threshold/baseline tracking, and blocks acceptance
on regression. Frontend displays coverage in gate/review panels with
a "Collect Coverage" button. Includes 20 Rust tests, 17 Vitest tests,
and 14 Playwright E2E tests.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/ReviewPanel.tsx

@@ -278,6 +278,22 @@ export function ReviewPanel({
 								Summary: {story.summary.passed}/{story.summary.total} passing,{" "}
 								{` ${story.summary.failed}`} failing
 							</div>
+							{story.coverage_report && (
+								<div
+									style={{
+										fontSize: "0.85em",
+										color:
+											story.coverage_report.current_percent <
+											story.coverage_report.threshold_percent
+												? "#ff7b72"
+												: "#7ee787",
+									}}
+								>
+									Coverage: {story.coverage_report.current_percent.toFixed(1)}%
+									(threshold:{" "}
+									{story.coverage_report.threshold_percent.toFixed(1)}%)
+								</div>
+							)}
 							{story.missing_categories.length > 0 && (
 								<div style={{ fontSize: "0.85em", color: "#ffb86c" }}>
 									Missing: {story.missing_categories.join(", ")}