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-lock.json

@@ -10,6 +10,7 @@
         "@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",
@@ -1247,6 +1248,33 @@
         "react": "^18 || ^19"
       }
     },
+    "node_modules/@tanstack/react-virtual": {
+      "version": "3.14.3",
+      "resolved": "https://registry.npmjs.org/@tanstack/react-virtual/-/react-virtual-3.14.3.tgz",
+      "integrity": "sha512-k/cnHPVaOfn46hSbiY6n4Dzf4QjCGWSF40zR5QIIYUqPAjpA6TN7InfYmcMiDVQGP2iUn9xsRbAl8u1v3UmeVQ==",
+      "license": "MIT",
+      "dependencies": {
+        "@tanstack/virtual-core": "3.17.1"
+      },
+      "funding": {
+        "type": "github",
+        "url": "https://github.com/sponsors/tannerlinsley"
+      },
+      "peerDependencies": {
+        "react": "^16.8.0 || ^17.0.0 || ^18.0.0 || ^19.0.0",
+        "react-dom": "^16.8.0 || ^17.0.0 || ^18.0.0 || ^19.0.0"
+      }
+    },
+    "node_modules/@tanstack/virtual-core": {
+      "version": "3.17.1",
+      "resolved": "https://registry.npmjs.org/@tanstack/virtual-core/-/virtual-core-3.17.1.tgz",
+      "integrity": "sha512-VZyW2Uiml5tmBZwPGrSD3Sz73OxzljQMCmzYHsUTPEuTsERf5xwa+uWb01xEzkz3ZSYTjj8NEb/mKHvgKxyZdA==",
+      "license": "MIT",
+      "funding": {
+        "type": "github",
+        "url": "https://github.com/sponsors/tannerlinsley"
+      }
+    },
     "node_modules/@types/babel__core": {
       "version": "7.20.5",
       "resolved": "https://registry.npmjs.org/@types/babel__core/-/babel__core-7.20.5.tgz",