feat: document upgrade stacking behaviour as multiplicative

Adds an inline note to the upgrade panel and updates the About
panel's Upgrades entry to explicitly state that multipliers stack
multiplicatively (two ×2 upgrades give ×4, not ×3).

Closes #60

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

@@ -31,7 +31,10 @@ const howToPlay = [
     body:
       "Purchase upgrades to multiply the gold and essence output of specific"
       + " adventurer tiers, or boost your whole guild. Upgrades are permanent"
-      + " for the current run and compound with each other.",
+      + " for the current run and stack multiplicatively — two ×2 upgrades"
+      + " targeting the same adventurer combine to give ×4, not ×3. Global"
+      + " upgrades multiply on top of adventurer-specific ones, so stacking"
+      + " both types compounds the effect significantly.",
     title: "🔧 Upgrades",
   },
   {

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

@@ -261,6 +261,10 @@ const UpgradePanel = (): JSX.Element => {
         {upgrades.length}
         {" purchased"}
       </p>
+      <p className="upgrade-stacking-note">
+        {"💡 Upgrade multipliers stack multiplicatively — two ×2 upgrades"
+          + " combine to give ×4, not ×3."}
+      </p>
       {upgrades.length === 0
         ? <p className="empty-state">
           {"No upgrades available yet — keep adventuring!"}