fix(timeline): scroll to top on section / folder switch

Clicking a folder in the sidebar (or any section change) didn't
reset the timeline's scroll position. If the user was scrolled deep
into All Photos, the new folder loaded at the same y-offset, often
landing on empty space below the last row.

The section-change effect already cleared selection and reset the
auto-focus guard; just needed to also reset parentRef.current.scrollTop.
Synchronous so the first paint of the new section is anchored at
photo[0]; the auto-focus selectPhoto call still runs after render
to highlight the first photo.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/components/timeline/Timeline.tsx

@@ -247,6 +247,18 @@ export function Timeline() {
     prevSectionRef.current = currentSection
     didAutoFocusRef.current = false
     clearSelection()
+    // Reset scroll to the top of the new section. Without this the
+    // previous section's scroll offset (which can be tens of thousands
+    // of px in a long timeline) carries over to a freshly-loaded
+    // folder, leaving the user looking at empty space below the last
+    // row. The auto-focus effect below selects photos[0] on next
+    // render, but its "ensure visible" scroll only kicks in if the
+    // cell is out of view — by then the user has already seen the
+    // wrong scroll position. Doing this synchronously here keeps the
+    // first paint of the new section anchored at the top.
+    if (parentRef.current) {
+      parentRef.current.scrollTop = 0
+    }
   }, [currentSection, clearSelection])
 
   // Membership in the active heap (drives the green tint on each