feat(layout): unified Inbox + UserMenu extraction

Replaces the topbar's separate AlertBell + NotificationBell with a
single Inbox popover that tabs between alerts and notifications.
NotificationBell keeps a popover-gate so it doesn't fire its list
fetch when Inbox is mounted alongside it.

Extracts the user dropdown into <UserMenu> and moves the port
switcher + role label + theme toggle into the sidebar footer so
the topbar can reclaim space for breadcrumbs and command search.

Adds dedicated Insights / Receipts nav sections in the sidebar
(scaffolds the website-analytics + upload-receipts entry points).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/components/notifications/notification-bell.tsx

@@ -1,5 +1,6 @@
 'use client';
 
+import { useState } from 'react';
 import { Bell } from 'lucide-react';
 import { useMutation, useQuery, useQueryClient } from '@tanstack/react-query';
 
@@ -27,11 +28,18 @@ interface NotificationListResponse {
 export function NotificationBell() {
   const { unreadCount } = useNotifications();
   const queryClient = useQueryClient();
+  // Track popover open state so we only fire the list fetch when the user
+  // actually opens the bell. Without this, an instance of NotificationBell
+  // mounted alongside <Inbox /> would populate the same ['notifications',
+  // 'list'] cache key without the gating Inbox carefully applies, defeating
+  // Inbox's enabled-on-open optimization.
+  const [open, setOpen] = useState(false);
 
   const { data, isLoading } = useQuery<NotificationListResponse>({
     queryKey: ['notifications', 'list'],
     queryFn: () => apiFetch('/api/v1/notifications?limit=20'),
     staleTime: 30_000,
+    enabled: open,
   });
 
   const markReadMutation = useMutation({
@@ -52,7 +60,7 @@ export function NotificationBell() {
   const notifications = data?.data ?? [];
 
   return (
-    <Popover>
+    <Popover open={open} onOpenChange={setOpen}>
       <PopoverTrigger asChild>
         <Button variant="ghost" size="icon" className="relative">
           <Bell className="h-5 w-5" />