feat(filters): move date picker to sidebar, track visible photo order

Pulls the date range picker out of the filter-bar pill into a
dedicated always-visible section at the top of the left sidebar, and
teaches the timeline to publish its visible photo sequence so "open
first photo" shortcuts respect the on-screen order.

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

frontend/src/components/layout/RightSidebar.tsx

@@ -213,6 +213,7 @@ export function RightSidebar() {
     </div>
   )
 
+
   if (selectedPhotos.length === 0) {
     return (
       <div