fix: ensure X-App-Source header survives per-request header overrides

Add onRequest hook to Nova $fetch client so X-App-Source: nova is
always injected even when individual calls pass their own headers
object (which shallow-replaces the createFetch defaults).

packages/lib/api.ts

@@ -254,6 +254,11 @@ export const $fetch = createFetch({
 	baseURL: `${process.env.NEXT_PUBLIC_BACKEND_URL ?? "https://api.supermemory.ai"}/v3`,
 	credentials: "include",
 	headers: { "X-App-Source": "nova" },
+	onRequest: (context: { headers: Headers }) => {
+		if (!context.headers.has("X-App-Source")) {
+			context.headers.set("X-App-Source", "nova")
+		}
+	},
 	retry: {
 		attempts: 3,
 		delay: 100,