fix: include admin_basepath in absolute navigation paths

Fixes navigate(..., { absolute: true }) only prepending basepath but
missing admin_basepath, causing links like /data instead of /admin/data.

Adds admin_basepath to BkndAdminConfig type.

Amp-Thread-ID: https://ampcode.com/threads/T-019cfd9f-89ca-703c-a86c-99eef60e3fe8
Co-authored-by: Amp <amp@ampcode.com>

app/src/ui/Admin.tsx

@@ -16,6 +16,11 @@ export type BkndAdminConfig = {
     * @default `/`
     */
    basepath?: string;
+   /**
+    * Sub-path for the Admin UI within the base path
+    * @default ``
+    */
+   admin_basepath?: string;
    /**
     * Path to return to when clicking the logo
     * @default `/`

app/src/ui/lib/routes.ts

@@ -101,7 +101,7 @@ export function useNavigate() {
                }
             }
 
-            const _url = options?.absolute ? `~/${basepath}${url}`.replace(/\/+/g, "/") : url;
+            const _url = options?.absolute ? `~/${app.options.basepath}/${app.options.admin_basepath}${url}`.replace(/\/+/g, "/") : url;
             const state = {
                ...options?.state,
                referrer: location,