Add better-auth integration and update dependencies in package.json and bun.lock

apps/web/src/lib/auth.ts

@@ -0,0 +1,6 @@
+import { createAuthClient } from "better-auth/svelte";
+import { convexClient } from "@convex-dev/better-auth/client/plugins";
+
+export const authClient = createAuthClient({
+  plugins: [convexClient()],
+});