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/button.jsx

@@ -0,0 +1,34 @@
+import { forwardRef } from 'react';
+import { cva } from 'class-variance-authority';
+import { cn } from '../../lib/utils.js';
+
+const buttonVariants = cva(
+  'inline-flex items-center justify-center gap-2 whitespace-nowrap rounded-md text-sm font-medium transition-colors focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-ring focus-visible:ring-offset-1 focus-visible:ring-offset-background disabled:pointer-events-none disabled:opacity-50 [&_svg]:size-4 [&_svg]:shrink-0',
+  {
+    variants: {
+      variant: {
+        primary: 'bg-primary text-primary-foreground hover:bg-primary/90 shadow-sm',
+        secondary: 'bg-muted text-foreground hover:bg-muted/70',
+        outline: 'border border-border bg-card hover:bg-muted text-foreground',
+        ghost: 'hover:bg-muted text-foreground',
+        danger: 'bg-danger text-danger-foreground hover:bg-danger/90 shadow-sm',
+        'danger-outline': 'border border-danger/40 text-danger hover:bg-danger/10',
+        link: 'text-primary underline-offset-4 hover:underline',
+      },
+      size: {
+        sm: 'h-8 px-3 text-xs',
+        md: 'h-9 px-4',
+        lg: 'h-10 px-6',
+        icon: 'h-9 w-9',
+        'icon-sm': 'h-8 w-8',
+      },
+    },
+    defaultVariants: { variant: 'primary', size: 'md' },
+  }
+);
+
+const Button = forwardRef(function Button({ className, variant, size, ...props }, ref) {
+  return <button ref={ref} className={cn(buttonVariants({ variant, size }), className)} {...props} />;
+});
+
+export { Button, buttonVariants };