feat: add Stats tab with task distribution and execution health metrics

- Export computeTaskStats and computeExecutionStats from app.js
- Add renderStatsPanel with state count grid, KPI row (total/success-rate/cost/avg-duration), and outcome bar chart
- Wire stats tab into switchTab and poll for live refresh
- Add Stats tab button and panel to index.html
- Add CSS for .stats-counts, .stats-kpis, .stats-bar-chart using existing state color variables
- Add docs/stats-tab-plan.md with component structure and data flow
- 14 new unit tests in web/test/stats.test.mjs (140 total, all passing)

No backend changes — derives all metrics from existing /api/tasks and /api/executions endpoints.

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

1 files changed, 159 insertions, 0 deletions

diff --git a/web/style.css b/web/style.css
index 1b2b344..feedcce 100644
--- a/web/style.css
+++ b/web/style.css
@@ -1199,3 +1199,162 @@ dialog label select:focus {
   margin-top: 0.25rem;
   text-align: right;
 }
+
+/* ── Stats tab ────────────────────────────────────────────────────────────── */
+
+[data-panel="stats"] {
+  padding: 1rem 1.5rem;
+  max-width: 900px;
+}
+
+.stats-section {
+  margin-bottom: 2rem;
+}
+
+.stats-section h2 {
+  font-size: 1rem;
+  font-weight: 600;
+  color: var(--text-muted);
+  text-transform: uppercase;
+  letter-spacing: 0.05em;
+  margin: 0 0 0.75rem;
+}
+
+/* Task state count boxes */
+.stats-counts {
+  display: flex;
+  flex-wrap: wrap;
+  gap: 0.5rem;
+}
+
+.stats-count-box {
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  padding: 0.75rem 1.25rem;
+  border-radius: 8px;
+  background: var(--card-bg);
+  border: 1px solid var(--border);
+  min-width: 80px;
+}
+
+.stats-count-box[data-state="RUNNING"]         { border-color: var(--state-running); }
+.stats-count-box[data-state="QUEUED"]          { border-color: var(--state-queued); }
+.stats-count-box[data-state="READY"]           { border-color: var(--state-running); }
+.stats-count-box[data-state="BLOCKED"]         { border-color: var(--state-blocked); }
+.stats-count-box[data-state="PENDING"]         { border-color: var(--state-pending); }
+.stats-count-box[data-state="COMPLETED"]       { border-color: var(--state-completed); }
+.stats-count-box[data-state="FAILED"]          { border-color: var(--state-failed); }
+.stats-count-box[data-state="TIMED_OUT"]       { border-color: var(--state-timed-out); }
+.stats-count-box[data-state="CANCELLED"]       { border-color: var(--state-cancelled); }
+.stats-count-box[data-state="BUDGET_EXCEEDED"] { border-color: var(--state-budget-exceeded); }
+
+.stats-count-number {
+  font-size: 1.75rem;
+  font-weight: 700;
+  line-height: 1;
+}
+
+.stats-count-label {
+  font-size: 0.7rem;
+  text-transform: uppercase;
+  letter-spacing: 0.04em;
+  color: var(--text-muted);
+  margin-top: 0.25rem;
+  white-space: nowrap;
+}
+
+/* KPI row */
+.stats-kpis {
+  display: grid;
+  grid-template-columns: repeat(4, 1fr);
+  gap: 0.75rem;
+  margin-bottom: 1.25rem;
+}
+
+@media (max-width: 600px) {
+  .stats-kpis {
+    grid-template-columns: repeat(2, 1fr);
+  }
+}
+
+.stats-kpi-box {
+  display: flex;
+  flex-direction: column;
+  align-items: center;
+  padding: 1rem;
+  border-radius: 8px;
+  background: var(--card-bg);
+  border: 1px solid var(--border);
+  text-align: center;
+}
+
+.stats-kpi-value {
+  font-size: 1.5rem;
+  font-weight: 700;
+  line-height: 1;
+}
+
+.stats-kpi-label {
+  font-size: 0.75rem;
+  color: var(--text-muted);
+  margin-top: 0.25rem;
+  white-space: nowrap;
+}
+
+/* Outcome bar chart */
+.stats-bar-chart-label {
+  font-size: 0.8rem;
+  color: var(--text-muted);
+  margin: 0 0 0.5rem;
+}
+
+.stats-bars {
+  display: flex;
+  flex-direction: column;
+  gap: 0.4rem;
+}
+
+.stats-bar-row {
+  display: flex;
+  align-items: center;
+  gap: 0.5rem;
+}
+
+.stats-bar-row-label {
+  font-size: 0.78rem;
+  text-transform: uppercase;
+  color: var(--text-muted);
+  width: 120px;
+  flex-shrink: 0;
+  white-space: nowrap;
+}
+
+.stats-bar-track {
+  flex: 1;
+  height: 16px;
+  background: var(--border);
+  border-radius: 4px;
+  overflow: hidden;
+}
+
+.stats-bar-fill {
+  height: 100%;
+  border-radius: 4px;
+  background: var(--state-pending);
+  transition: width 0.3s ease;
+}
+
+.stats-bar-fill[data-state="COMPLETED"]       { background: var(--state-completed); }
+.stats-bar-fill[data-state="FAILED"]          { background: var(--state-failed); }
+.stats-bar-fill[data-state="CANCELLED"]       { background: var(--state-cancelled); }
+.stats-bar-fill[data-state="TIMED_OUT"]       { background: var(--state-timed-out); }
+.stats-bar-fill[data-state="BUDGET_EXCEEDED"] { background: var(--state-budget-exceeded); }
+.stats-bar-fill[data-state="RUNNING"]         { background: var(--state-running); }
+
+.stats-bar-count {
+  font-size: 0.75rem;
+  color: var(--text-muted);
+  width: 80px;
+  flex-shrink: 0;
+}