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.ts

@@ -0,0 +1,6 @@
+import { betterAuth } from "better-auth";
+
+export const auth = betterAuth({
+  secret: process.env.BETTER_AUTH_SECRET,
+  url: process.env.BETTER_AUTH_URL
+});