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

convex/auth.config.ts

@@ -0,0 +1,6 @@
+import { getAuthConfigProvider } from "@convex-dev/better-auth/auth-config";
+import type { AuthConfig } from "convex/server";
+
+export default {
+  providers: [getAuthConfigProvider()],
+} satisfies AuthConfig;