polish: clarify parks open badge; improve timezone display

- "parks open" → "parks open this week" for clarity
- Week calendar cells: stack hours above tz abbreviation (smaller,
  dimmer) instead of inline to avoid overflow in tight 130px columns
- Mobile park cards: tz abbreviation inline but smaller/dimmer (60% opacity)
- Month calendar: same two-line stacking in compact day cells

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/page.tsx

@@ -135,7 +135,7 @@ export default async function HomePage({ searchParams }: PageProps) {
             color: visibleParks.length > 0 ? "var(--color-open-hours)" : "var(--color-text-muted)",
             fontWeight: 600,
           }}>
-            {visibleParks.length} of {PARKS.length} parks open
+            {visibleParks.length} of {PARKS.length} parks open this week
           </span>
         </div>