fix(pdf): stop a flowing day stranding its header at the foot of a page

Keeping a stay whole moved its cards to the next sheet, and the header bar
stayed behind on the old one — empty, above a stretch of white, with the
same day printed again overhead. The repeat itself is the continuation
marker from #1471; what was wrong is that there was nothing left to
continue from.

break-after on the thead and break-before on the tbody are both ignored by
Chromium here, measured rather than assumed. Holding the whole day together
is what works: a day that fits moves as one, and a day too long for a page
still breaks and still repeats its header. Scoped to the flowing layout,
since a day that starts its own page can never strand one.

client/src/components/PDF/TripPDF.test.ts

@@ -1014,4 +1014,20 @@ describe('page breaks between days (#1292)', () => {
     const rule = html.slice(html.indexOf('.day-accommodation {'))
     expect(rule.slice(0, rule.indexOf('}'))).toContain('break-inside: avoid')
   })
+
+  // Measured against Chromium: with the days flowing, a header bar could be left
+  // at the foot of a sheet while its content moved on, and the repeat from #1471
+  // then read as the same day printed twice. break-after on the thead and
+  // break-before on the tbody are both ignored; holding the day together is what
+  // works, and a day too long for one page still breaks and still repeats.
+  it('FE-PDF-BREAK-008: a flowing day is held together so its header is never stranded', async () => {
+    localStorage.setItem('trek_pdf_page_break_per_day', '0')
+    await downloadTripPDF(twoDays)
+    const html = getIframe()!.srcdoc
+
+    expect(html).toContain('.pdf-flow .day-section { break-inside: avoid; page-break-inside: avoid; }')
+    // Scoped: a day that starts its own page cannot strand a header.
+    const at = html.indexOf('.day-section { break-inside: avoid')
+    expect(html.slice(at - 10, at)).toContain('.pdf-flow ')
+  })
 })

client/src/components/PDF/TripPDF.tsx

@@ -542,6 +542,15 @@ export async function downloadTripPDF({ trip, days, places, assignments = {}, ca
   .day-break { page-break-before: always; }
   .pdf-flow .day-break { page-break-before: auto; }
   .pdf-flow .day-section + .day-section { margin-top: 18px; }
+  /* Hold a flowing day together. Without this the header bar can be placed at the
+     foot of a sheet while its content moves to the next one, which then repeats
+     the header (#1471) and reads as the same day printed twice. A day too long
+     for one page still breaks and still repeats its header — the engine only
+     honours this where it can. Days that start a page of their own cannot strand
+     a header, so it is scoped to the flowing layout.
+     Measured, not assumed: break-after on the thead and break-before on the tbody
+     are both ignored by Chromium here. */
+  .pdf-flow .day-section { break-inside: avoid; page-break-inside: avoid; }
   .day-section { width: 100%; border-collapse: collapse; table-layout: fixed; }
   .day-header-bar {
     background: #0f172a; padding: 11px 28px;