feat(ui): F1+F2 — component primitives + rebuilt app shell

F1: shadcn-style primitive set in components/ui (Button, Card, Badge, Input,
Switch, Separator, Skeleton, Tooltip, DropdownMenu, Dialog, Sheet, Tabs, Table,
Toast+useToast, ThemeToggle, PageHeader/StatCard/EmptyState) on Radix + cva.

F2: Layout rebuilt on the new system — sidebar with primary nav (lucide icons)
+ favorites/saved-searches/mailbox/smart-folder sections, sticky topbar with
search, sync status, theme toggle, digest toggle, and an account dropdown. All
prior behavior preserved (auth, sync, collapse/section persistence, "/" search
focus). App wrapped in ToastProvider + TooltipProvider. Build green.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/components/ui/index.js

@@ -0,0 +1,48 @@
+// Barrel export for the UI primitive set. Import from '../components/ui'.
+export { Button, buttonVariants } from './button.jsx';
+export { Card, CardHeader, CardTitle, CardDescription, CardContent, CardFooter } from './card.jsx';
+export { Badge, badgeVariants } from './badge.jsx';
+export { Input, Textarea } from './input.jsx';
+export { Switch } from './switch.jsx';
+export { Separator } from './separator.jsx';
+export { Skeleton } from './skeleton.jsx';
+export { Tooltip, TooltipTrigger, TooltipContent, TooltipProvider } from './tooltip.jsx';
+export {
+  DropdownMenu,
+  DropdownMenuTrigger,
+  DropdownMenuContent,
+  DropdownMenuItem,
+  DropdownMenuCheckboxItem,
+  DropdownMenuLabel,
+  DropdownMenuSeparator,
+  DropdownMenuGroup,
+  DropdownMenuSub,
+  DropdownMenuSubContent,
+  DropdownMenuSubTrigger,
+  DropdownMenuRadioGroup,
+} from './dropdown-menu.jsx';
+export {
+  Dialog,
+  DialogTrigger,
+  DialogClose,
+  DialogContent,
+  DialogHeader,
+  DialogFooter,
+  DialogTitle,
+  DialogDescription,
+} from './dialog.jsx';
+export {
+  Sheet,
+  SheetTrigger,
+  SheetClose,
+  SheetContent,
+  SheetHeader,
+  SheetFooter,
+  SheetTitle,
+  SheetDescription,
+} from './sheet.jsx';
+export { Tabs, TabsList, TabsTrigger, TabsContent } from './tabs.jsx';
+export { Table, TableHeader, TableBody, TableRow, TableHead, TableCell } from './table.jsx';
+export { ToastProvider, useToast } from './toast.jsx';
+export { default as ThemeToggle } from './theme-toggle.jsx';
+export { PageHeader, StatCard, EmptyState } from './misc.jsx';