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.spec.tsx

@@ -98,6 +98,27 @@ describe("AppLayout", () => {
     pushMock.mockClear();
   });
 
+  it("starts with a skip link that moves the focus to the page content", async () => {
+    render(
+      <AppLayout>
+        <main>
+          <p>Seiteninhalt</p>
+        </main>
+      </AppLayout>,
+    );
+
+    const link = screen.getByRole("link", { name: "Zum Inhalt springen" });
+    // First in the tab order: before the sidebar and every other link.
+    await userEvent.tab();
+    expect(link).toHaveFocus();
+    expect(link).toHaveAttribute("href", "#main-content");
+
+    await userEvent.keyboard("{Enter}");
+    const content = screen.getByText("Seiteninhalt").closest("#main-content");
+    expect(content).not.toBeNull();
+    expect(content).toHaveFocus();
+  });
+
   it("leaves room under the page for the quick-add button on a phone, so it covers no action", () => {
     pathname = "/suggestions";
     permissions = ["inspections:create"];