perf+ux: cheaper sidebar toggle, virtualised filmstrip, compact toasts

Timeline's items array used to rebuild on every sub-pixel cellSize tick
during the sidebar CSS transition, causing visible jank with thousands
of photos. Row heights now resolve off a ref at virtualizer-measure
time, so items only rebuild when the column count actually changes.
PreviewFilmstrip is horizontally virtualised (~15 cells in the DOM
instead of N), cutting preview open latency on large libraries. Also
honor the user's explicit right-sidebar collapse (don't auto-reopen on
photo selection) and shrink the sonner toasts to a tighter form factor.

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

frontend/src/App.tsx

@@ -29,6 +29,11 @@ import { TooltipProvider } from '@/components/ui/tooltip'
 function MainApp() {
   const [leftSidebarOpen, setLeftSidebarOpen] = useState(true)
   const [rightSidebarOpen, setRightSidebarOpen] = useState(true)
+  // Respect the user's manual collapse of the metadata panel. Once they
+  // close it explicitly (via `i` hotkey or the sidebar toggle button),
+  // selecting a new photo should NOT force it back open. Cleared when
+  // they open it manually again.
+  const rightCollapsedByUser = useRef(false)
   const viewMode = usePhotoStore((state) => state.viewMode)
   const activePhotoId = usePhotoStore((state) => state.activePhotoId)
   const currentSection = useFilterStore((s) => s.currentSection)
@@ -45,9 +50,24 @@ function MainApp() {
   }, [currentSection])
 
   useEffect(() => {
-    setRightSidebarOpen(!!activePhotoId)
+    if (!activePhotoId) {
+      setRightSidebarOpen(false)
+      return
+    }
+    // User explicitly collapsed the panel — don't undo that just because
+    // they picked a different photo.
+    if (rightCollapsedByUser.current) return
+    setRightSidebarOpen(true)
   }, [activePhotoId])
 
+  const toggleRightSidebar = () => {
+    setRightSidebarOpen((prev) => {
+      const next = !prev
+      rightCollapsedByUser.current = !next
+      return next
+    })
+  }
+
   // Bidirectional sync of filter store with URL query params.
   useFilterUrlSync()
 
@@ -59,7 +79,7 @@ function MainApp() {
   // Set up global keyboard shortcuts
   useKeyboardShortcuts({
     onToggleLeftSidebar: () => setLeftSidebarOpen(!leftSidebarOpen),
-    onToggleRightSidebar: () => setRightSidebarOpen(!rightSidebarOpen),
+    onToggleRightSidebar: toggleRightSidebar,
     getFirstPhotoId: () => allPhotos?.[0]?.id ?? null,
   })
 
@@ -96,7 +116,7 @@ function MainApp() {
               leftSidebarOpen={leftSidebarOpen}
               rightSidebarOpen={showRightSidebar}
               onToggleLeftSidebar={() => setLeftSidebarOpen(!leftSidebarOpen)}
-              onToggleRightSidebar={() => setRightSidebarOpen(!rightSidebarOpen)}
+              onToggleRightSidebar={toggleRightSidebar}
             />
           )}
           {!isSettings && <DiscardActionBar />}