feat: sticky month-header overlay in Timeline

The inline date headers can't use CSS position: sticky because
TanStack Virtual positions every item with transform translateY,
which removes them from the document flow.

Workaround: render a separate overlay above the scroll container
that's absolutely positioned (left/right/top: 0) and updates its
label as the user scrolls. The current label is computed from a
pre-built headerOffsets array (cumulative sum of item heights up
to each header) — find the latest header whose offset <= scrollTop,
and that's the group containing whatever's at the top of the view.

The overlay sits at z-20 above the photos with bg-bg/90 +
backdrop-blur and pointer-events-none so it doesn't intercept
clicks. Inline headers still render so the visual flow at group
boundaries is smooth — the overlay is the persistent label.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/timeline/Timeline.tsx

@@ -145,6 +145,20 @@ export function Timeline() {
     [photos, columns, sortBy]
   )
 
+  // Pre-computed offset of every header in the virtualizer's coordinate
+  // space, used to drive the sticky-header overlay below.
+  const headerOffsets = useMemo(() => {
+    const result: { offset: number; label: string }[] = []
+    let cumulative = 0
+    for (const item of items) {
+      if (item.type === 'header') {
+        result.push({ offset: cumulative, label: item.label })
+      }
+      cumulative += item.height
+    }
+    return result
+  }, [items])
+
   // Range-selection helper. Operates on the global photos array, not on
   // virtualizer items.
   const selectRange = (endIndex: number) => {
@@ -171,6 +185,32 @@ export function Timeline() {
     virtualizer.measure()
   }, [items, virtualizer])
 
+  // Track scroll position so we can show the current group label as a
+  // pinned overlay at the top of the scroll container. The virtualizer's
+  // items use transform translateY (so CSS position: sticky doesn't work
+  // on the inline headers); the overlay sidesteps that by living outside
+  // the virtualizer's positioned children.
+  const [scrollTop, setScrollTop] = useState(0)
+  useEffect(() => {
+    const el = parentRef.current
+    if (!el) return
+    const onScroll = () => setScrollTop(el.scrollTop)
+    el.addEventListener('scroll', onScroll, { passive: true })
+    return () => el.removeEventListener('scroll', onScroll)
+  }, [])
+
+  // Find the latest header whose start <= scrollTop. That's the label of
+  // the group containing whatever is currently at the top of the viewport.
+  const stickyLabel = useMemo(() => {
+    if (headerOffsets.length === 0) return null
+    let current: string | null = null
+    for (const h of headerOffsets) {
+      if (h.offset <= scrollTop) current = h.label
+      else break
+    }
+    return current
+  }, [headerOffsets, scrollTop])
+
   // Measure container width on mount and resize.
   useEffect(() => {
     const measureWidth = () => {
@@ -267,11 +307,23 @@ export function Timeline() {
   }
 
   return (
-    <div
-      ref={parentRef}
-      className="h-full overflow-auto bg-bg"
-      style={{ padding: `${PADDING}px` }}
-    >
+    <div className="relative h-full">
+      {/* Sticky group-header overlay. Lives outside the virtualizer's
+       *  positioned children so it isn't affected by translateY transforms.
+       *  Updates as the user scrolls past month boundaries. */}
+      {stickyLabel && (
+        <div className="pointer-events-none absolute left-0 right-0 top-0 z-20 border-b border-border bg-bg/90 px-4 py-1 backdrop-blur-sm">
+          <h3 className="text-sm font-semibold uppercase tracking-wide text-text">
+            {stickyLabel}
+          </h3>
+        </div>
+      )}
+
+      <div
+        ref={parentRef}
+        className="h-full overflow-auto bg-bg"
+        style={{ padding: `${PADDING}px` }}
+      >
       <div
         style={{
           height: `${virtualizer.getTotalSize()}px`,
@@ -342,6 +394,7 @@ export function Timeline() {
           )
         })}
       </div>
+      </div>
     </div>
   )
 }