feat(dashboard): align base refresh to wall-clock minute boundary

Instead of a fixed 60s interval from an arbitrary start offset, the base status
refresh now fires on each wall-clock minute boundary (:00), keeping the
minute-precision timestamp shown in the message accurate. Event-driven updates
(new message / agent activity) still refresh in between. Adds
msUntilNextMinuteBoundary with tests.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/unified-dashboard.test.ts

@@ -6,6 +6,7 @@ import {
   createCoalescingTrigger,
   editStatusMessageWithRetry,
   formatStatusHeader,
+  msUntilNextMinuteBoundary,
   getDashboardDuplicateCleanupIntervalMs,
   renderUsageTable,
   shouldPurgeDashboardChannelOnStart,
@@ -349,6 +350,15 @@ describe('editStatusMessageWithRetry', () => {
   });
 });
 
+describe('msUntilNextMinuteBoundary', () => {
+  it('returns time to the next :00 boundary', () => {
+    expect(msUntilNextMinuteBoundary(120_000)).toBe(60_000); // exactly on boundary
+    expect(msUntilNextMinuteBoundary(120_000 + 15_000)).toBe(45_000); // 15s in
+    expect(msUntilNextMinuteBoundary(120_000 + 59_000)).toBe(1_000); // 59s in
+    expect(msUntilNextMinuteBoundary(120_000 + 1)).toBe(59_999); // 1ms in
+  });
+});
+
 describe('createCoalescingTrigger', () => {
   it('coalesces a burst into one run and re-arms after firing', () => {
     let runs = 0;

src/unified-dashboard.ts

@@ -840,6 +840,16 @@ export function createCoalescingTrigger(
   };
 }
 
+/**
+ * Milliseconds from `nowMs` until the next wall-clock minute boundary (:00).
+ * Minute boundaries align across timezones (offsets are whole minutes), so this
+ * is timezone-agnostic. Exactly on a boundary returns a full minute.
+ */
+export function msUntilNextMinuteBoundary(nowMs: number): number {
+  const rem = nowMs % 60_000;
+  return rem === 0 ? 60_000 : 60_000 - rem;
+}
+
 export async function startUnifiedDashboard(
   opts: UnifiedDashboardOptions,
 ): Promise<void> {
@@ -989,7 +999,15 @@ export async function startUnifiedDashboard(
     );
   }
 
-  setInterval(updateStatus, opts.statusUpdateInterval);
+  // Base periodic refresh fires on each wall-clock minute boundary (:00) so the
+  // minute-precision timestamp shown in the status message stays accurate.
+  const scheduleMinuteBoundaryUpdate = () => {
+    setTimeout(() => {
+      void updateStatus();
+      scheduleMinuteBoundaryUpdate();
+    }, msUntilNextMinuteBoundary(Date.now()));
+  };
+  scheduleMinuteBoundaryUpdate();
   setInterval(() => {
     if (!isRenderer || !statusMessageId) return;
     void cleanupDashboardDuplicateMessages(opts, statusMessageId);