feat(frontend): group dashboard sidebar navigation into sections

Add section-based grouping to the sidebar navigation with headers and
dividers. Menu items are now organized into 4 sections:
- Allgemein (Übersicht, Zählpunkte, Mein Profil)
- Community (Community beitreten, Meine Mitgliedschaften, Verbrauch)
- Tarife & Einladungen (Meine Tarife, Einladungen)
- Verwaltung (Admin-only: Mitgliedschaften, Benutzerverwaltung, etc.)

Only sections with visible items for the current role are rendered.
All 107 frontend tests passing.

eeg_frontend/src/app/layout/dashboard-layout/dashboard-layout.html

@@ -21,15 +21,25 @@
       </a>
     </div>
 
-    <nav class="flex-1 px-4 py-6 space-y-1 overflow-y-auto">
-      @for (item of visibleNavItems(); track item.path) {
-        <a [routerLink]="item.path"
-           routerLinkActive="bg-emerald-50 text-emerald-700 font-medium"
-           [routerLinkActiveOptions]="{exact: item.path === '/dashboard'}"
-           (click)="closeMobileMenu()"
-           class="block px-4 py-2.5 rounded-lg text-slate-600 hover:bg-slate-50 hover:text-slate-900 transition-colors">
-          {{ item.label }}
-        </a>
+    <nav class="flex-1 px-4 py-6 space-y-4 overflow-y-auto">
+      @for (group of navSections(); track group.section; let last = $last) {
+        <div>
+          <p class="px-4 mb-1 text-xs font-semibold uppercase tracking-wider text-slate-400">
+            {{ group.section }}
+          </p>
+          @for (item of group.items; track item.path) {
+            <a [routerLink]="item.path"
+               routerLinkActive="bg-emerald-50 text-emerald-700 font-medium"
+               [routerLinkActiveOptions]="{exact: item.path === '/dashboard'}"
+               (click)="closeMobileMenu()"
+               class="block px-4 py-2 rounded-lg text-slate-600 hover:bg-slate-50 hover:text-slate-900 transition-colors">
+              {{ item.label }}
+            </a>
+          }
+        </div>
+        @if (!last) {
+          <div class="border-t border-slate-100 mx-4"></div>
+        }
       }
     </nav>
   </aside>

eeg_frontend/src/app/layout/dashboard-layout/dashboard-layout.ts

@@ -1,7 +1,7 @@
 import {Component, computed, inject, signal} from '@angular/core';
 import {Router, RouterLink, RouterLinkActive, RouterOutlet} from '@angular/router';
 import {AuthService} from '../../services/auth';
-import {NAV_ITEMS} from './dashboard-nav-items';
+import {NAV_ITEMS, NAV_SECTIONS, NavItem} from './dashboard-nav-items';
 import {NotificationComponent} from '../../components/notification/notification';
 
 @Component({
@@ -17,9 +17,17 @@ export class DashboardLayout {
 
   isMobileMenuOpen = signal(false);
 
-  visibleNavItems = computed(() => {
+  navSections = computed(() => {
     const role = this.authService.currentUserRole();
-    return NAV_ITEMS.filter(item => role !== null && item.roles.includes(role));
+    if (role === null) return [];
+
+    const visible = NAV_ITEMS.filter(item => item.roles.includes(role));
+    return NAV_SECTIONS
+        .map(section => ({
+          section,
+          items: visible.filter(item => item.section === section)
+        }))
+        .filter(group => group.items.length > 0);
   });
 
   toggleMobileMenu() {

eeg_frontend/src/app/layout/dashboard-layout/dashboard-nav-items.ts

@@ -2,21 +2,31 @@ export interface NavItem {
   label: string;
   path: string;
   roles: string[];
+  section: string;
 }
 
+export const NAV_SECTIONS = ['Allgemein', 'Community', 'Tarife & Einladungen', 'Verwaltung'] as const;
+
 export const NAV_ITEMS: NavItem[] = [
-  {label: 'Übersicht', path: '/dashboard', roles: ['ADMIN', 'MEMBER']},
-  {label: 'Zählpunkte', path: '/dashboard/metering-points', roles: ['ADMIN', 'MEMBER']},
-  {label: 'Community beitreten', path: '/dashboard/join-community', roles: ['MEMBER']},
-  {label: 'Meine Mitgliedschaften', path: '/dashboard/my-memberships', roles: ['MEMBER']},
-  {label: 'Verbrauch', path: '/dashboard/consumption', roles: ['MEMBER']},
-  {label: 'Mitgliedschaften', path: '/dashboard/admin-memberships', roles: ['ADMIN']},
-  {label: 'Mein Profil', path: '/dashboard/profile', roles: ['ADMIN', 'MEMBER']},
-  {label: 'Benutzerverwaltung', path: '/dashboard/approvals', roles: ['ADMIN']},
-  {label: 'Energiegemeinschaften', path: '/dashboard/energy-communities', roles: ['ADMIN']},
-  {label: 'Tarife verwalten', path: '/dashboard/tariffs', roles: ['ADMIN']},
-  {label: 'Messdaten-Upload', path: '/dashboard/upload-metering-data', roles: ['ADMIN']},
-  {label: 'Meine Tarife', path: '/dashboard/my-tariffs', roles: ['MEMBER']},
-  {label: 'Meine Einladungen', path: '/dashboard/producer-invites', roles: ['MEMBER']},
-  {label: 'Offene Einladungen', path: '/dashboard/consumer-invites', roles: ['MEMBER']}
+  // Allgemein
+  {label: 'Übersicht', path: '/dashboard', roles: ['ADMIN', 'MEMBER'], section: 'Allgemein'},
+  {label: 'Zählpunkte', path: '/dashboard/metering-points', roles: ['ADMIN', 'MEMBER'], section: 'Allgemein'},
+  {label: 'Mein Profil', path: '/dashboard/profile', roles: ['ADMIN', 'MEMBER'], section: 'Allgemein'},
+
+  // Community
+  {label: 'Community beitreten', path: '/dashboard/join-community', roles: ['MEMBER'], section: 'Community'},
+  {label: 'Meine Mitgliedschaften', path: '/dashboard/my-memberships', roles: ['MEMBER'], section: 'Community'},
+  {label: 'Verbrauch', path: '/dashboard/consumption', roles: ['MEMBER'], section: 'Community'},
+
+  // Tarife & Einladungen
+  {label: 'Meine Tarife', path: '/dashboard/my-tariffs', roles: ['MEMBER'], section: 'Tarife & Einladungen'},
+  {label: 'Meine Einladungen', path: '/dashboard/producer-invites', roles: ['MEMBER'], section: 'Tarife & Einladungen'},
+  {label: 'Offene Einladungen', path: '/dashboard/consumer-invites', roles: ['MEMBER'], section: 'Tarife & Einladungen'},
+
+  // Verwaltung
+  {label: 'Mitgliedschaften', path: '/dashboard/admin-memberships', roles: ['ADMIN'], section: 'Verwaltung'},
+  {label: 'Benutzerverwaltung', path: '/dashboard/approvals', roles: ['ADMIN'], section: 'Verwaltung'},
+  {label: 'Energiegemeinschaften', path: '/dashboard/energy-communities', roles: ['ADMIN'], section: 'Verwaltung'},
+  {label: 'Tarife verwalten', path: '/dashboard/tariffs', roles: ['ADMIN'], section: 'Verwaltung'},
+  {label: 'Messdaten-Upload', path: '/dashboard/upload-metering-data', roles: ['ADMIN'], section: 'Verwaltung'},
 ];