feat: replace sidebar with 21st.dev hover-to-expand component

Use framer-motion animated sidebar from 21st.dev — collapses to icons,
expands on hover. Phosphor icons instead of lucide. Remove old manual
collapse/expand and sidebar-context. Top bar has Pulse logo + user
actions, sidebar below with site picker and nav groups.

package.json

@@ -21,6 +21,7 @@
     "@tanstack/react-virtual": "^3.13.21",
     "@types/d3": "^7.4.3",
     "class-variance-authority": "^0.7.1",
+    "clsx": "^2.1.1",
     "cobe": "^0.6.5",
     "country-flag-icons": "^1.6.4",
     "d3": "^7.9.0",
@@ -40,6 +41,7 @@
     "sonner": "^2.0.7",
     "svg-dotted-map": "^2.0.1",
     "swr": "^2.3.3",
+    "tailwind-merge": "^3.5.0",
     "xlsx": "^0.18.5"
   },
   "overrides": {