feat(ui): add avg-by-hour panel to dashboard stats

Complements the existing avg-by-weekday chart with its orthogonal
partner: which hour of the day the user typically commits. The api
buckets events by EXTRACT(hour FROM occurred_at AT TIME ZONE $tz) so
the chart matches the clock the user sees rather than UTC; the UI
passes the browser's resolved IANA timezone. Renders as 24 mini-bars
below the weekday chart with labels every 4 hours and per-bar
tooltips showing the average events/day at that hour.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ui/src/components/ContributionStats.tsx

@@ -1,7 +1,7 @@
 import { useMemo } from 'react';
 import { useQuery } from '@tanstack/react-query';
 
-import { fetchDailyCounts, fetchSources } from '../api/client';
+import { fetchDailyCounts, fetchHourlyAvgs, fetchSources } from '../api/client';
 
 export function ContributionStats() {
   const sourcesQ = useQuery({
@@ -31,6 +31,21 @@ export function ContributionStats() {
     staleTime: 10 * 60_000,
   });
 
+  // Bucket hour-of-day in the user's local timezone so the chart matches
+  // the clock they see. Browser may report e.g. "Europe/Helsinki"; fall
+  // back to UTC if the resolver returns something the server won't
+  // accept (it validates the string before binding).
+  const tz = useMemo(
+    () => Intl.DateTimeFormat().resolvedOptions().timeZone || 'UTC',
+    [],
+  );
+  const hourlyQ = useQuery({
+    queryKey: ['hourly-avgs-alltime', fromStr, toStr, tz],
+    queryFn: () => fetchHourlyAvgs(fromStr, toStr, tz),
+    enabled: !!earliest,
+    staleTime: 10 * 60_000,
+  });
+
   const stats = useMemo(() => {
     const counts = dailyQ.data ?? [];
     if (counts.length === 0) return null;
@@ -96,6 +111,17 @@ export function ContributionStats() {
     return { currentStreak, longestStreak, busiest, dayAvgs, maxAvg, activeDays };
   }, [dailyQ.data]);
 
+  const hourly = useMemo(() => {
+    const data = hourlyQ.data ?? [];
+    if (data.length === 0) return null;
+    const byHour = new Array(24).fill(0);
+    for (const { hour, avg } of data) {
+      if (hour >= 0 && hour < 24) byHour[hour] = avg;
+    }
+    const max = Math.max(...byHour);
+    return { hours: byHour, max };
+  }, [hourlyQ.data]);
+
   if (!stats) return null;
 
   return (
@@ -139,6 +165,31 @@ export function ContributionStats() {
             ))}
           </div>
         </div>
+        {hourly && (
+          <div className="mt-3">
+            <span style={{ opacity: 0.7, fontSize: '0.75rem' }}>avg by hour ({tz})</span>
+            <div className="d-flex align-items-end gap-1 mt-1" style={{ height: 64 }}>
+              {hourly.hours.map((avg, h) => (
+                <div key={h} className="d-flex flex-column align-items-center" style={{ flex: 1, height: '100%', justifyContent: 'flex-end' }}>
+                  <div style={{ width: '100%', borderRadius: 2, background: 'rgba(255,255,255,0.05)', flex: 1, display: 'flex', flexDirection: 'column', justifyContent: 'flex-end' }}>
+                    <div
+                      style={{
+                        height: hourly.max > 0 ? `${(avg / hourly.max) * 100}%` : '0%',
+                        borderRadius: 2,
+                        backgroundColor: '#39d353',
+                        opacity: 0.7,
+                      }}
+                      title={`${h.toString().padStart(2, '0')}:00 — ${avg.toFixed(2)}/day`}
+                    />
+                  </div>
+                  <span style={{ fontSize: '0.6rem', opacity: 0.7, marginTop: 2, minHeight: '0.7rem' }}>
+                    {h % 4 === 0 ? h.toString().padStart(2, '0') : ''}
+                  </span>
+                </div>
+              ))}
+            </div>
+          </div>
+        )}
       </div>
     </div>
   );