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>

.gitignore

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+# dependencies
+/node_modules
+/.pnp
+.pnp.js
+
+# testing
+/coverage
+
+# next.js
+/.next/
+/out/
+
+# production
+/build
+
+# misc
+.DS_Store
+*.pem
+
+# debug
+npm-debug.log*
+yarn-debug.log*
+yarn-error.log*
+
+# local env files
+.env*.local
+.env
+
+# vercel
+.vercel
+
+# typescript
+*.tsbuildinfo
+next-env.d.ts