docs(trips): say whose money the trip total counts

Total Spend is every payer's trip-tagged spending; the split figures directly
below it are scoped to the owner. Two lenses on one screen read as one unless
the card says which it is. The number is unchanged and deliberate -- a trip
cost what the group put into it -- so this is a label, not a fix.

src/app/trips/[id]/page.tsx

@@ -128,7 +128,10 @@ export default function TripDetailPage({ params }: { params: Promise<{ id: strin
 
       {/* Stat cards */}
       <div className="grid grid-cols-2 sm:grid-cols-4 gap-4">
-        <StatCard label="Total Spend" value={`$${Number(total_spend).toFixed(2)}`} sub="all transactions" color={t.color} />
+        {/* Deliberately every payer, not just this owner — a trip cost what the
+            group put into it. The split figures below are owner-scoped, so this
+            says whose money it counts to stop the two being read as one lens. */}
+        <StatCard label="Total Spend" value={`$${Number(total_spend).toFixed(2)}`} sub="all payers, net of refunds" color={t.color} />
         <StatCard label="Transactions" value={String(transaction_count)} sub="total" color={t.color} />
         <StatCard label="Daily Average" value={`$${Number(daily_average).toFixed(2)}`} sub="per day" color={t.color} />
         <StatCard label="Days" value={String(num_days)} sub={dateRange ?? "date range"} color={t.color} />