feat: dashboard, Convex auth, UI components, and LemonSpace branding

- Add dashboard shell with auth integration
- Wire Better Auth / Convex (client, server, HTTP routes)
- Add shadcn-style UI primitives and logo assets
- Update global styles and landing page
- Add internal docs (.docs)

Made-with: Cursor

lib/auth-client.ts

@@ -0,0 +1,7 @@
+import { createAuthClient } from "better-auth/react";
+import { convexClient } from "@convex-dev/better-auth/client/plugins";
+
+// Next.js: kein crossDomainClient nötig (same-origin via API Route Proxy)
+export const authClient = createAuthClient({
+  plugins: [convexClient()],
+});