#301: stop /settings scrolling horizontally at 320px

WCAG 2.1 SC 1.4.10 asks for no two-dimensional scrolling down to 320px,
which is also what 400% zoom on a 1280px screen produces. The layout
skeleton was already hardened for this in #165; the overflow came from
content inside the sections.

- The sessions table cannot shrink below its min-content width — four
  columns, one of them the full user-agent string. It now scrolls inside
  its own container rather than pushing the page. The container is
  focusable with a role and a name, because a scroll area that only a
  mouse can reach trades one barrier for another.
- `.settings-checkbox` rows may wrap. The accent swatches have a fixed
  size and cannot shrink, so an unwrappable row set a floor for the whole
  page width.

Adds a reflow guard to the a11y pack. axe does not cover 1.4.10 — the
criterion is not derivable from the DOM — so this is a separate check,
and it names the overflowing elements when it trips instead of only
reporting that something overflows.

apps/web/e2e/a11y.spec.ts

@@ -104,3 +104,55 @@ for (const scheme of SCHEMES) {
     });
   });
 }
+
+/**
+ * Reflow (WCAG 2.1 SC 1.4.10, issue #301): bei 320 px CSS-Breite — was 400 %
+ * Zoom auf 1280 px entspricht — darf die Seite nicht seitenweit horizontal
+ * scrollen. axe prüft das NICHT, das Kriterium ist nicht maschinell aus dem
+ * DOM ableitbar; deshalb ein eigener Zaun.
+ *
+ * Schlägt er an, nennt er die überstehenden Elemente. Ohne diese Diagnose
+ * weiß man nur DASS es überläuft und muss im Browser bisektieren.
+ */
+const NARROW = { width: 320, height: 800 };
+
+async function expectNoHorizontalScroll(page: Page, label: string): Promise<void> {
+  const report = await page.evaluate(() => {
+    const doc = document.documentElement;
+    const limit = doc.clientWidth;
+    const offenders: string[] = [];
+    for (const el of Array.from(document.querySelectorAll<HTMLElement>('body *'))) {
+      const rect = el.getBoundingClientRect();
+      // 1 px Toleranz gegen Subpixel-Rundung.
+      if (rect.width > 0 && rect.right > limit + 1) {
+        const cls =
+          el.className && typeof el.className === 'string'
+            ? `.${el.className.trim().split(/\s+/).join('.')}`
+            : '';
+        offenders.push(
+          `${el.tagName.toLowerCase()}${cls} (right=${Math.round(rect.right)}, width=${Math.round(rect.width)})`,
+        );
+      }
+    }
+    return { scrollWidth: doc.scrollWidth, clientWidth: limit, offenders: offenders.slice(0, 12) };
+  });
+  expect(
+    { overflowBy: report.scrollWidth - report.clientWidth, offenders: report.offenders },
+    `${label}: horizontaler Überlauf bei 320 px`,
+  ).toEqual({ overflowBy: 0, offenders: [] });
+}
+
+test.describe('reflow at 320px', () => {
+  test('user settings do not scroll horizontally at 320px', async ({ browser }) => {
+    const context = await contextForUser(browser, BASE, 'fixture-user');
+    const page = await context.newPage();
+    await page.setViewportSize(NARROW);
+    await page.goto('/settings');
+    await page.waitForLoadState('networkidle');
+    // Die Sitzungstabelle rendert asynchron und ist der breiteste Inhalt —
+    // ohne sie misst der Zaun eine halb aufgebaute Seite.
+    await page.locator('.table tbody tr').first().waitFor();
+    await expectNoHorizontalScroll(page, '/settings');
+    await context.close();
+  });
+});

apps/web/src/pages/SettingsPage.tsx

@@ -385,6 +385,18 @@ function SessionsSection(): React.JSX.Element {
   return (
     <section className="settings-section">
       <h2>{t('settings:sessions.title')}</h2>
+      {/* A table cannot shrink below its min-content width, so the user-agent
+          column pushed the whole page into horizontal scrolling at 320px
+          (issue #301, WCAG 1.4.10). It scrolls inside its own box instead —
+          the content stays reachable, which `overflow: hidden` would not. */}
+      <div
+        className="table-scroll"
+        // A scroll container is only operable by keyboard once it is
+        // focusable; role+name keep it from being an unlabelled stop.
+        tabIndex={0}
+        role="region"
+        aria-label={t('settings:sessions.title')}
+      >
         <table className="table">
           <thead>
             <tr>
@@ -401,7 +413,9 @@ function SessionsSection(): React.JSX.Element {
               <tr key={session.id}>
                 <td>
                   {session.userAgent ?? '—'}
-                {session.current && <span className="badge">{t('settings:sessions.current')}</span>}
+                  {session.current && (
+                    <span className="badge">{t('settings:sessions.current')}</span>
+                  )}
                 </td>
                 <td>{formatTime(session.createdAt)}</td>
                 <td>{formatTime(session.lastSeenAt)}</td>
@@ -420,6 +434,7 @@ function SessionsSection(): React.JSX.Element {
             ))}
           </tbody>
         </table>
+      </div>
       {others.length > 0 ? (
         <button type="button" className="button" onClick={() => revokeOthers.mutate()}>
           {t('settings:sessions.revokeAll')}

apps/web/src/styles/base.css

@@ -966,6 +966,14 @@ button {
   border-bottom: 1px solid var(--color-border);
 }
 
+/* Wrapper for tables that can outgrow a narrow viewport (issue #301). The
+   table keeps its own scrollbar; `tabindex` makes that scroll area reachable
+   by keyboard, which a bare overflow container is not. */
+.table-scroll {
+  overflow-x: auto;
+  max-width: 100%;
+}
+
 .table {
   width: 100%;
   border-collapse: collapse;
@@ -3761,6 +3769,10 @@ ul[data-type='task_list'] li p:last-of-type {
   align-items: center;
   gap: var(--space-2);
   margin: var(--space-2) 0;
+  /* Radio + label + accent swatches must be allowed to break onto a second
+     line at 320px (issue #301, WCAG 1.4.10) — the swatches have a fixed size
+     and cannot shrink, so without this the row sets a floor for the page. */
+  flex-wrap: wrap;
 }
 
 /* Radio groups in settings sections (issue #180). */