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>

crates/moments-entities/src/lib.rs

@@ -91,6 +91,14 @@ pub struct DailyCount {
     pub count: i64,
 }
 
+/// Average events per day at a given hour of the day, computed in a
+/// caller-supplied IANA timezone. 24 entries (0..=23).
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct HourlyAvg {
+    pub hour: i32,
+    pub avg: f64,
+}
+
 /// Per-repo activity rollup for the dashboard.
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct ProjectSummary {