Show match finish times in the browser timezone

The history table truncated finished_at, which the API serializes in
UTC, so the Finished column always showed UTC wall time. Parse the
timestamp with the browser's Date and render its local components,
keeping the existing YYYY-MM-DD HH:MM layout.

web/src/model.rs

@@ -291,6 +291,13 @@ pub fn fmt_points(points: f64) -> String {
     }
 }
 
+/// Render a calendar instant as `YYYY-MM-DD HH:MM`. `month` is 0-based
+/// (the JavaScript `Date` convention); the other components are as
+/// returned by `Date`'s local-time getters.
+pub fn fmt_datetime_parts(year: u32, month: u32, day: u32, hour: u32, minute: u32) -> String {
+    format!("{year:04}-{:02}-{day:02} {hour:02}:{minute:02}", month + 1)
+}
+
 /// The caller's Elo rating for one game type (GET /api/me/ratings).
 #[derive(Debug, Clone, Deserialize)]
 #[allow(dead_code)]
@@ -658,6 +665,12 @@ mod tests {
         assert_eq!("2.5", fmt_points(2.5));
     }
 
+    #[test]
+    fn fmt_datetime_parts_pads_and_offsets_month() {
+        assert_eq!("2026-09-19 05:02", fmt_datetime_parts(2026, 8, 19, 5, 2));
+        assert_eq!("2026-01-01 00:00", fmt_datetime_parts(2026, 0, 1, 0, 0));
+    }
+
     #[test]
     fn leaderboard_points_accept_floats() {
         let e: LeaderboardEntry = serde_json::from_value(json!({