Remove donation_amounts feature flag from DonationsBox

The only frontend experiment - drop the useFeatureValue hook and use the
DONATIONS_BOX_VALUES constant directly at all three reference sites.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: aapeliv

app/web/features/donations/DonationsBox.tsx

@@ -13,7 +13,6 @@ import { useMutation } from "@tanstack/react-query";
 import Alert from "components/Alert";
 import Button from "components/Button";
 import StyledLink from "components/StyledLink";
-import { useFeatureValue } from "experimentation";
 import {
   DONATIONS_BOX_CURRENCY,
   DONATIONS_BOX_VALUES,
@@ -218,11 +217,6 @@ interface DonationFormData {
 export default function DonationsBox() {
   const { t } = useTranslation(DONATIONS);
 
-  const donationAmounts = useFeatureValue<number[]>(
-    "donation_amounts",
-    DONATIONS_BOX_VALUES,
-  );
-
   const [isPredefinedAmount, setIsPredefinedAmount] = useState(true);
 
   const router = useRouter();
@@ -355,11 +349,11 @@ export default function DonationsBox() {
       <Controller
         name="amount"
         control={control}
-        defaultValue={donationAmounts[2]}
+        defaultValue={DONATIONS_BOX_VALUES[2]}
         render={({ field }) => (
           <AmountGrid>
             {[
-              ...donationAmounts.map((value) => {
+              ...DONATIONS_BOX_VALUES.map((value) => {
                 return (
                   <StyledAmountButton
                     key={value}
@@ -386,7 +380,7 @@ export default function DonationsBox() {
                     field.onChange(
                       typeof e.target.valueAsNumber === "number"
                         ? e.target.valueAsNumber
-                        : donationAmounts[0],
+                        : DONATIONS_BOX_VALUES[0],
                     );
                     setIsPredefinedAmount(false);
                   }}