fix: T6 browser-check findings (compliance, treatments, labels)

- api-client accepts 2xx answers without body (revoke showed an error)
- authority notices: sentOn in the future is rejected (Berlin day) on
  create, mark-sent and PATCH; dialog defaults to the Berlin day
- service worker never caches /api/v1/site-settings
- treatments/new no longer overflows at 390 px (min-w-0 on form
  wrappers); dose with more than 3 decimals is an error, never rounded
- medicines: expired lots flagged, disposal over the rest warns
- label designer: side panel at lg, sample values instead of
  placeholders, compliance panel reopens on lg, labelled zoom buttons
- skip link to #main-content, headings on no-access pages

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

ApiCore-Frontend/src/app/(app)/layout.tsx

@@ -24,6 +24,9 @@ import { BottomNav } from "./bottom-nav";
 import { hasQuickAddButton, QuickAddSheet } from "./quick-add-sheet";
 import { PageTransition } from "./page-transition";
 
+/** Id of the page content, the target of the skip link. */
+const MAIN_CONTENT_ID = "main-content";
+
 export default function AppLayout({ children }: { children: ReactNode }) {
   const router = useRouter();
   const pathname = usePathname();
@@ -69,6 +72,18 @@ export default function AppLayout({ children }: { children: ReactNode }) {
   // `position: sticky` and the StickySubmit save bar would never stick.
   return (
     <div className="flex min-h-screen pr-(--safe-area-right) pl-(--safe-area-left)">
+      <a
+        href={`#${MAIN_CONTENT_ID}`}
+        onClick={(event) => {
+          // The hash alone would not move the focus into a Next.js page reliably.
+          event.preventDefault();
+          pageContentRef.current?.focus();
+          pageContentRef.current?.scrollIntoView?.({ block: "start" });
+        }}
+        className="sr-only focus:not-sr-only focus:fixed focus:top-2 focus:left-2 focus:z-50 focus:rounded-lg focus:bg-card focus:px-4 focus:py-3 focus:text-sm focus:font-medium focus:shadow-[var(--shadow-elevated)] focus:ring-3 focus:ring-ring/50 focus:outline-none"
+      >
+        Zum Inhalt springen
+      </a>
       <Sidebar user={user} />
       <div className="flex min-w-0 flex-1 flex-col">
         <header className="sticky top-0 z-10 flex items-center justify-between gap-3 bg-card/85 px-4 py-3 shadow-[var(--shadow-soft)] backdrop-blur-md sm:justify-end sm:px-6">
@@ -117,6 +132,7 @@ export default function AppLayout({ children }: { children: ReactNode }) {
           syncEnabled={sessionMode === "online"}
         />
         <div
+          id={MAIN_CONTENT_ID}
           ref={pageContentRef}
           tabIndex={-1}
           className={`flex-1 ${contentPaddingClass} outline-none sm:pb-(--safe-area-bottom)`}