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

app/api/auth/[...all]/route.ts

@@ -0,0 +1,3 @@
+import { handler } from "@/lib/auth-server";
+
+export const { GET, POST } = handler;