feat: Initial Hub Dashboard with email, calendar, and tasks

AI-powered unified business workspace for LetsBe Cloud tenants.
- Next.js 15 (App Router) with Keycloak SSO via NextAuth.js v5
- Email client: IMAP/SMTP via ImapFlow + nodemailer (inbox, compose, search, folders)
- Calendar: CalDAV via tsdav + Schedule-X (month/week/day views, event CRUD)
- Task management: Vikunja API integration (list view, kanban, filters)
- Responsive shell with sidebar navigation and activity feed
- Docker deployment ready

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.env.example

@@ -0,0 +1,29 @@
+# Authentication
+NEXTAUTH_URL=https://hub.example.com
+NEXTAUTH_SECRET=generate-a-secret-here
+
+# Keycloak SSO
+KEYCLOAK_CLIENT_ID=hub-dashboard
+KEYCLOAK_CLIENT_SECRET=your-client-secret
+KEYCLOAK_ISSUER=https://auth.example.com/realms/letsbe
+
+# Orchestrator API (same server)
+ORCHESTRATOR_URL=http://orchestrator:8100
+
+# Tenant domain (set during provisioning)
+TENANT_DOMAIN=example.com
+
+# Server IP
+SERVER_IP=0.0.0.0
+
+# IMAP/SMTP (Poste.io on same server)
+IMAP_HOST=mail.example.com
+IMAP_PORT=993
+SMTP_HOST=mail.example.com
+SMTP_PORT=587
+
+# CalDAV (Nextcloud on same server)
+CALDAV_URL=https://cloud.example.com/remote.php/dav
+
+# Vikunja Task Manager
+VIKUNJA_URL=https://tasks.example.com/api/v1