feat: add frontend dashboard with traffic lights, timeline, and AI summary (Phase 2L)

Dashboard page at /dashboard with 5 components:
- DeadlineTrafficLights: RED/AMBER/GREEN cards with animated counts and pulse for overdue
- CaseOverviewGrid: active/new/closed case counts
- UpcomingTimeline: merged deadlines + appointments for next 7 days, grouped by day
- AISummaryCard: natural language summary generated from dashboard data
- QuickActions: shortcuts to create cases, deadlines, AI analysis, CalDAV sync

3-column responsive grid layout. Root / redirects to /dashboard.
Fetches from GET /api/dashboard with 60s auto-refresh via react-query.

frontend/src/components/layout/Sidebar.tsx

@@ -12,7 +12,7 @@ import {
 } from "lucide-react";
 
 const navigation = [
-  { name: "Dashboard", href: "/", icon: LayoutDashboard },
+  { name: "Dashboard", href: "/dashboard", icon: LayoutDashboard },
   { name: "Akten", href: "/akten", icon: FolderOpen },
   { name: "Fristen", href: "/fristen", icon: Clock },
   { name: "Termine", href: "/termine", icon: Calendar },
@@ -30,10 +30,7 @@ export function Sidebar() {
       </div>
       <nav className="flex-1 space-y-0.5 p-2">
         {navigation.map((item) => {
-          const isActive =
-            item.href === "/"
-              ? pathname === "/"
-              : pathname.startsWith(item.href);
+          const isActive = pathname.startsWith(item.href);
           return (
             <Link
               key={item.href}