feat: Add Inbox view with task/appointment/email management

- Add InboxView.vue with full CRUD UI
- Add inbox to navigation (first item)
- Support filtering by status/type
- Show stats (due today, overdue, etc.)
- Quick status updates with checkbox
- Priority indicators and overdue warnings

src/layouts/AppLayout.vue

@@ -10,6 +10,7 @@ const sidebarOpen = ref(false)
 const isMobile = ref(false)
 
 const baseNavItems = [
+  { name: 'Inbox', path: '/inbox', icon: 'inbox' },
   { name: 'Dashboard', path: '/', icon: 'chart-pie' },
   { name: 'Kontakte', path: '/contacts', icon: 'users' },
   { name: 'Firmen', path: '/companies', icon: 'building-office' },
@@ -138,6 +139,9 @@ function isActive(path) {
           @click="closeSidebarOnMobile"
         >
           <!-- Icons -->
+          <svg v-if="item.icon === 'inbox'" class="w-5 h-5 flex-shrink-0" fill="none" viewBox="0 0 24 24" stroke="currentColor">
+            <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M20 13V6a2 2 0 00-2-2H6a2 2 0 00-2 2v7m16 0v5a2 2 0 01-2 2H6a2 2 0 01-2-2v-5m16 0h-2.586a1 1 0 00-.707.293l-2.414 2.414a1 1 0 01-.707.293h-3.172a1 1 0 01-.707-.293l-2.414-2.414A1 1 0 006.586 13H4" />
+          </svg>
           <svg v-if="item.icon === 'chart-pie'" class="w-5 h-5 flex-shrink-0" fill="none" viewBox="0 0 24 24" stroke="currentColor">
             <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M11 3.055A9.001 9.001 0 1020.945 13H11V3.055z" />
             <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M20.488 9H15V3.512A9.025 9.025 0 0120.488 9z" />