feat: Quiet Luxury design system + Pipeline (ex-Lead) redesign

- Design system foundations: Plus Jakarta Sans font, shadow-card/radius
  tokens in @theme, design-reference/DESIGN-SYSTEM.md with component inventory
- Collapsible admin shell (AdminShell): smooth w-64<->w-20 sidebar, new top
  header with "Admin" + avatar placeholder, localStorage-persisted state
- Rename route /admin/leads -> /admin/pipeline (redirect stubs preserved),
  nav label Lead -> Pipeline; DB table unchanged
- Reusable primitives: StatusBadge, SearchInput, SegmentedToggle (dual-theme)
- Luxury restyle of lead table, kanban (6 stages + @dnd-kit intact), PageHeader
- Tokenize editable-cell / option-multi-select for dark-mode legibility

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/components/admin/PageHeader.tsx

@@ -6,10 +6,10 @@ export function PageHeader({ title, subtitle, action }: {
   action?: ReactNode;
 }) {
   return (
-    <div className="mb-6 flex items-start justify-between gap-4">
+    <div className="mb-6 flex flex-col sm:flex-row items-start sm:items-center justify-between gap-4">
       <div>
-        <h1 className="text-2xl font-bold text-[#1a1a1a]">{title}</h1>
-        {subtitle && <p className="mt-1 text-sm text-[#71717a]">{subtitle}</p>}
+        <h1 className="text-2xl font-semibold tracking-tight text-foreground">{title}</h1>
+        {subtitle && <p className="mt-1 text-xs text-muted-foreground">{subtitle}</p>}
       </div>
       {action}
     </div>