fix(shared): keep the header's decoration from widening the page

The two decorative circles in the header are positioned outside it on purpose
(-40 to -60 on each axis) and nothing in the ancestor chain clips them, so the
first one widened the document by its own overhang: a 390px phone reported a
450px scrollWidth and the whole page could be dragged sideways (#2345).

overflow: hidden on the header is what the decoration always meant.

client/src/pages/SharedTripPage.test.tsx

@@ -1450,4 +1450,19 @@ describe('SharedTripPage', () => {
       expect(document.querySelector('a[href^="javascript:"]')).toBeNull();
     });
   });
+
+  // ── #2345: the header has to clip the decoration it bleeds ──────────────
+
+  describe('FE-PAGE-SHARED-040: the header clips the circles it bleeds (#2345)', () => {
+    it('does not let them widen the page', async () => {
+      await open('overflow-token', payload({}));
+
+      // Both circles sit outside the header on purpose, so the page only stays
+      // as wide as the viewport if the header itself is the clip. The header is
+      // the first gradient panel on the page and carries the trip title.
+      const header = document.querySelector<HTMLElement>('div[style*="linear-gradient(135deg"]');
+      expect(header?.textContent).toContain('Shared Paris Trip');
+      expect(header?.style.overflow).toBe('hidden');
+    });
+  });
 });

client/src/pages/SharedTripPage.tsx

@@ -205,6 +205,12 @@ export default function SharedTripPage() {
           padding: '32px 20px 28px',
           textAlign: 'center',
           position: 'relative',
+          // The two circles below bleed past this box on purpose (-40 to -60 on
+          // each axis) and nothing else on the page clips them, so without this
+          // the header widened the document by 60px: 390 + 60 = the 450px
+          // scrollWidth a 390px phone reported, and the whole page could be
+          // dragged sideways (#2345).
+          overflow: 'hidden',
         }}
       >
         {/* Cover image background */}