fix(journey): stop the load-more observer rebuilding itself on every render

ScrollTrigger kept onVisible in its effect dependencies, and both callers hand
in a fresh arrow on every render: the picker its loadMorePhotos, the gallery
its setVisibleCount step. So every unrelated re-render, a photo being ticked
included, disconnected the IntersectionObserver and observed again, and each
observe() replays the initial callback.

The callback moves into a ref. loading stays a dependency, because re-observing
once a page has landed is the only thing that still produces a callback when
the sentinel never left the 200px margin, so folding that into a ref too would
leave paging dead after the first page. FE-COMP-JSCROLLTRIG-009 pins that, 007
and 008 pin the rebuild itself; both are red on the old component.

Markup, classes and the signature are untouched.

client/src/components/Journey/JourneyDetailPageScrollTrigger.test.tsx

@@ -1,4 +1,4 @@
-// FE-COMP-JSCROLLTRIG-001 to FE-COMP-JSCROLLTRIG-006
+// FE-COMP-JSCROLLTRIG-001 to FE-COMP-JSCROLLTRIG-009
 
 import { render } from '../../../tests/helpers/render'
 import { resetAllStores } from '../../../tests/helpers/store'
@@ -87,4 +87,44 @@ describe('ScrollTrigger', () => {
     unmount()
     expect(observers[0].disconnect).toHaveBeenCalled()
   })
+
+  it('FE-COMP-JSCROLLTRIG-007: a fresh callback identity alone does not rebuild the observer', () => {
+    // Both callers hand in a new arrow every render; rebuilding for that churn
+    // replays the initial callback and pulls pages nobody asked for.
+    const { rerender } = render(<ScrollTrigger onVisible={vi.fn()} loading={false} />)
+    expect(observers).toHaveLength(1)
+
+    rerender(<ScrollTrigger onVisible={vi.fn()} loading={false} />)
+    rerender(<ScrollTrigger onVisible={vi.fn()} loading={false} />)
+
+    expect(observers).toHaveLength(1)
+    expect(observers[0].disconnect).not.toHaveBeenCalled()
+  })
+
+  it('FE-COMP-JSCROLLTRIG-008: the surviving observer calls the newest callback, not the one it was built with', () => {
+    const first = vi.fn()
+    const second = vi.fn()
+    const { rerender } = render(<ScrollTrigger onVisible={first} loading={false} />)
+    rerender(<ScrollTrigger onVisible={second} loading={false} />)
+
+    intersect(observers[0], true)
+
+    expect(first).not.toHaveBeenCalled()
+    expect(second).toHaveBeenCalledTimes(1)
+  })
+
+  it('FE-COMP-JSCROLLTRIG-009: a settled page really re-observes the sentinel', () => {
+    // A browser observer only fires on a threshold crossing plus once per
+    // observe(). A sentinel that never leaves the 200px margin therefore only
+    // gets another callback because the loading flip rebuilds the observer, so
+    // loading must stay a dependency rather than moving into a ref as well.
+    const onVisible = vi.fn()
+    const { rerender } = render(<ScrollTrigger onVisible={onVisible} loading />)
+    expect(observers).toHaveLength(1)
+
+    rerender(<ScrollTrigger onVisible={onVisible} loading={false} />)
+
+    expect(observers).toHaveLength(2)
+    expect(observers[0].disconnect).toHaveBeenCalled()
+  })
 })

client/src/components/Journey/JourneyDetailPageScrollTrigger.tsx

@@ -2,13 +2,24 @@ import { useEffect, useRef } from 'react'
 
 export function ScrollTrigger({ onVisible, loading }: { onVisible: () => void; loading: boolean }) {
   const ref = useRef<HTMLDivElement>(null)
+  // Both callers hand in a fresh arrow on every render — the picker its
+  // loadMorePhotos, the gallery its setVisibleCount step. With onVisible in the
+  // effect deps, every unrelated re-render tore the observer down and observed
+  // again, and each observe() replays the initial callback. The ref keeps the
+  // observer out of that churn.
+  //
+  // loading stays a dependency on purpose: re-observing after a page has landed
+  // is the only thing that still produces a callback when the sentinel never
+  // left the 200px margin.
+  const onVisibleRef = useRef(onVisible)
+  onVisibleRef.current = onVisible
   useEffect(() => {
     const el = ref.current
     if (!el) return
-    const obs = new IntersectionObserver(([entry]) => { if (entry.isIntersecting && !loading) onVisible() }, { rootMargin: '200px' })
+    const obs = new IntersectionObserver(([entry]) => { if (entry.isIntersecting && !loading) onVisibleRef.current() }, { rootMargin: '200px' })
     obs.observe(el)
     return () => obs.disconnect()
-  }, [onVisible, loading])
+  }, [loading])
   return (
     <div ref={ref} className="flex justify-center py-4 mt-2">
       <div className="w-5 h-5 border-2 border-zinc-300 border-t-zinc-900 dark:border-zinc-600 dark:border-t-white rounded-full animate-spin" />