Make portal mobile-friendly with floating buttons and slide-over panels

Replace fixed sidebar with floating action buttons (menu + add node) in
bottom-left corner. Sidebar and node panel now slide in as overlay drawers
with backdrop dismiss, capped width for small screens, and slide animations.

portal/src/components/Sidebar.tsx

@@ -9,18 +9,23 @@ export default function Sidebar({
   selectedId,
   onSelect,
   onAddNode,
+  onClose,
 }: {
   selectedId: string | null;
   onSelect: (id: string) => void;
   onAddNode: () => void;
+  onClose: () => void;
 }) {
   const [kindFilter, setKindFilter] = useState('');
   const { data: nodes, isLoading } = useNodes(kindFilter ? { kind: kindFilter } : undefined);
 
   return (
-    <aside className="w-72 bg-gray-900 border-r border-gray-800 flex flex-col h-full">
+    <aside className="h-full bg-gray-900 border-r border-gray-800 flex flex-col">
       <div className="p-4 border-b border-gray-800">
-        <h1 className="text-lg font-bold text-white mb-3">Cortex Portal</h1>
+        <div className="flex items-center justify-between mb-3">
+          <h1 className="text-lg font-bold text-white">Cortex Portal</h1>
+          <button onClick={onClose} className="text-gray-500 hover:text-white text-xl leading-none">&times;</button>
+        </div>
         <SearchBar onSelect={onSelect} />
         <div className="flex gap-1 mt-3 flex-wrap">
           {KINDS.map((k) => (