fix(ui): say what the auto-router savings are measured against

"vs. the router's baseline model" defined the number by a config field most
operators never set, and now that the baseline is derived it named nothing at
all. The comparison is the priciest model the router can reach, so the card says
that.

ui/litellm-dashboard/src/app/(dashboard)/cost-optimization/_components/UsageTab.tsx

@@ -203,8 +203,8 @@ const UsageTab: React.FC<UsageTabProps> = ({ accessToken, activity }) => {
         <SummaryCard
           label="Auto-router savings"
           value={usd(autorouterTotal)}
-          hint="vs. the router's baseline model"
-          info="What this traffic would have cost on the router's baseline model, minus what it actually cost. Switching models leaves the new one with a cold cache, so the cache-write it pays counts against the saving; a negative total means routing cost more than staying on the baseline would have."
+          hint="vs. the priciest model it could pick"
+          info="What this traffic would have cost had every request gone to the most expensive model the auto-router can route to, minus what it actually cost."
         />
       </div>