feat: add milestone prestige bonuses every 5th prestige

Every 5th prestige awards a scaling runestone windfall:
milestone_number * 25 stones (prestige 5 = 25, 10 = 50, 50 = 250, etc).
Shown in the ascension success message when non-zero.

apps/web/src/components/game/PrestigePanel.tsx

@@ -42,7 +42,7 @@ export const PrestigePanel = (): React.JSX.Element => {
   const { state, reload, formatNumber, buyPrestigeUpgrade } = useGame();
   const [characterName, setCharacterName] = useState("");
   const [isPending, setIsPending] = useState(false);
-  const [result, setResult] = useState<{ runestones: number; count: number } | null>(null);
+  const [result, setResult] = useState<{ runestones: number; count: number; milestoneRunestones: number } | null>(null);
   const [prestigeError, setPrestigeError] = useState<string | null>(null);
   const [buyingId, setBuyingId] = useState<string | null>(null);
   const [activeTab, setActiveTab] = useState<"prestige" | "shop">("prestige");
@@ -65,7 +65,7 @@ export const PrestigePanel = (): React.JSX.Element => {
     setPrestigeError(null);
     try {
       const data = await prestige({ characterName: characterName.trim() });
-      setResult({ runestones: data.runestones, count: data.newPrestigeCount });
+      setResult({ runestones: data.runestones, count: data.newPrestigeCount, milestoneRunestones: data.milestoneRunestones });
       await reload();
     } catch (err) {
       setPrestigeError(err instanceof Error ? err.message : "Prestige failed");
@@ -176,6 +176,9 @@ export const PrestigePanel = (): React.JSX.Element => {
               {result && (
                 <p className="success">
                   Ascended to Prestige {result.count}! Earned {formatNumber(result.runestones)} Runestones.
+                  {result.milestoneRunestones > 0 && (
+                    <> 🎉 Milestone bonus: +{formatNumber(result.milestoneRunestones)} Runestones!</>
+                  )}
                 </p>
               )}
             </div>