fix(quote-response): compute minutes-remaining for the DE changeWithin string

Previous fix (45f0606) papered over the bug by changing the German
wording from "innerhalb von {{minutes}} Minuten" to "bis {{at}}" —
that worked but changed the UX intent. The original German wording
("you have N minutes left") was deliberate and clearer than an
absolute clock time; the actual bug was that no caller ever computed
`minutes` from `responseLockedAt`.

Revert the DE translation to its original wording, then build a
{ at, minutes } object at the call site so EN ("until {{at}}") and
DE ("innerhalb von {{minutes}} Minuten") each pick up the variable
they need. `minutes` rounds UP so a 14m 32s remainder displays as
"15 Minuten" rather than promising 14 the customer can't actually
hit.

frontend/src/i18n/locales/de.json

@@ -3798,7 +3798,7 @@
     "acceptedLocked": "Dieses Angebot wurde bereits angenommen.",
     "declinedLocked": "Dieses Angebot wurde abgelehnt.",
     "cannotRespond": "Es ist nicht mehr möglich, auf dieses Angebot zu antworten.",
-    "changeWithin": "Sie können Ihre Antwort bis {{at}} ändern.",
+    "changeWithin": "Sie können Ihre Antwort innerhalb von {{minutes}} Minuten ändern.",
     "intro": "Vielen Dank für Ihre Anfrage. Bitte prüfen Sie das folgende Angebot und antworten Sie unten.",
     "accept": "Angebot annehmen",
     "decline": "Angebot ablehnen"

frontend/src/pages/public/QuoteResponsePage.tsx

@@ -297,7 +297,22 @@ export const QuoteResponsePage: React.FC = () => {
               <>
                 <p className="text-sm text-neutral-600 dark:text-neutral-400 mb-4">
                   {quote.respondedAt
-                    ? t('quoteResponse.changeWithin', 'You can change your response until {{at}}.', { at: quote.responseLockedAt ? new Date(quote.responseLockedAt).toLocaleTimeString() : '' })
+                    ? t('quoteResponse.changeWithin', 'You can change your response until {{at}}.', (() => {
+                        // Provide both variables so EN ("until {{at}}")
+                        // and DE ("innerhalb von {{minutes}} Minuten")
+                        // each render correctly without needing locale-
+                        // specific call sites. `minutes` rounds UP so a
+                        // 14m 32s remainder shows as "15 Minuten" — never
+                        // promise a number the user can't actually hit.
+                        const lockAt = quote.responseLockedAt ? new Date(quote.responseLockedAt) : null;
+                        const minutes = lockAt
+                          ? Math.max(0, Math.ceil((lockAt.getTime() - Date.now()) / 60000))
+                          : 0;
+                        return {
+                          at: lockAt ? lockAt.toLocaleTimeString() : '',
+                          minutes,
+                        };
+                      })())
                     : t('quoteResponse.intro', 'Please accept or decline this quote:')}
                 </p>