feat(dashboard): local-time greeting + timezone-drift banner

Greeting
- The "Good morning / afternoon / evening, Matt" line now derives from the
  browser's local time, computed inside a useEffect so the rendered HTML
  can't lock to the server's clock during hydration. Until the effect
  fires, the header reads "Welcome" — a neutral phrase that's correct at
  every hour and never produces a hydration warning. The phrase re-evaluates
  hourly so a rep leaving the dashboard open across a boundary (5am, noon,
  6pm) doesn't keep stale text on screen.

Timezone-drift banner
- New <TimezoneDriftBanner> on the dashboard surfaces when the browser's
  resolved timezone (Intl.DateTimeFormat().resolvedOptions().timeZone, which
  follows the OS — and the OS usually follows physical location) doesn't
  match the user's stored CRM preference. The rep gets a one-tap "Update to
  Tokyo" button and a dismiss × that's sticky per browser via localStorage.
- Why a banner rather than auto-update: the stored timezone drives reminder
  firing time, daily-digest delivery, and due-date rendering. Silently
  pinning it to a transient travel location would shift their reminder
  schedule underfoot. The banner gives them control.

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

src/components/dashboard/activity-feed.tsx

@@ -131,7 +131,9 @@ function buildDiffLine(item: ActivityItem): string | null {
     if (keys.length === 0) return null;
     return keys
       .slice(0, 2)
-      .map((k) => `${humanizeFieldName(k)}: ${shortValue(oldObj[k], k)} → ${shortValue(newObj[k], k)}`)
+      .map(
+        (k) => `${humanizeFieldName(k)}: ${shortValue(oldObj[k], k)} → ${shortValue(newObj[k], k)}`,
+      )
       .join(' · ');
   }