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/App.tsx

@@ -76,11 +76,18 @@ function MainApp() {
   // of what filter key it's stored under.
   const { data: allPhotos } = usePhotosQuery()
 
-  // Set up global keyboard shortcuts
+  // Set up global keyboard shortcuts. Prefer the Timeline's published
+  // visible sequence (which respects per-month ordering) over the raw
+  // backend list — otherwise "Space on a blank selection" would open
+  // the globally first photo, which isn't what the user sees at the
+  // top-left of the grid.
   useKeyboardShortcuts({
     onToggleLeftSidebar: () => setLeftSidebarOpen(!leftSidebarOpen),
     onToggleRightSidebar: toggleRightSidebar,
-    getFirstPhotoId: () => allPhotos?.[0]?.id ?? null,
+    getFirstPhotoId: () =>
+      usePhotoStore.getState().visiblePhotoIds[0] ??
+      allPhotos?.[0]?.id ??
+      null,
   })
 
   // Settings page is a full-page section — hide filter bar, right sidebar,