feat: snappier timeline — instant discard, progressive load, restore preview origin

- Discard yanks photos from the grid optimistically (cache strip + active
  cursor advance) instead of waiting for the mutation round-trip; wired
  through the X hotkey, RightSidebar bulk discard, LeftSidebar discard
  drop, and DiscardActionBar restore/delete.
- usePhotosQuery resolves on the first 500-photo page and streams the
  remaining pages into the cache in the background, so the first
  thumbnails paint immediately on large libraries.
- Closing preview restores the photo it was opened on (snapshot ref in
  PreviewView, written directly to the store) and Timeline scrolls that
  row back into view. Escape is handled on the dialog with
  stopPropagation so Timeline's window-level Esc handler doesn't wipe
  the restored selection.
- Preview overlay bumped to z-[1000] so it covers Leaflet map tiles,
  and the right sidebar no longer collapses during preview — both fix
  visible layout shifts on close.

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

frontend/src/App.tsx

@@ -42,7 +42,11 @@ function App() {
 
   // Right sidebar stays open by default and shows whatever's selected
   // (or an empty state if nothing is). User can still toggle it manually.
-  const showRightSidebar = rightSidebarOpen && viewMode === 'grid'
+  // Note: deliberately NOT gated on viewMode — the preview overlay sits
+  // on top with z-[1000], so leaving the sidebar mounted underneath
+  // costs nothing visually and avoids the collapse-then-reopen layout
+  // shift the user would otherwise see every time they exit preview.
+  const showRightSidebar = rightSidebarOpen
 
   return (
     <div className="flex flex-col h-screen bg-bg text-text">