feat(feed): virtualize the feed list and consume the keyset cursor

Render only the visible rows via @tanstack/react-virtual (useVirtualizer
against the app's <main> scroll container, per-row dynamic measurement),
so the DOM no longer accumulates every loaded card. A ResizeObserver keeps
the responsive grid column count in sync and chunks cards into virtualized
rows; the list view virtualizes single-card rows. Infinite scroll now
triggers from the virtual range and pages via next_cursor instead of
offset; feed/count drops its unused paging args.

frontend/package.json

@@ -12,6 +12,7 @@
     "@dnd-kit/sortable": "^10.0.0",
     "@dnd-kit/utilities": "^3.2.2",
     "@tanstack/react-query": "^5.51.0",
+    "@tanstack/react-virtual": "^3.14.3",
     "clsx": "^2.1.1",
     "i18next": "^23.11.5",
     "lucide-react": "^0.408.0",