fix: normalize admin basepath to prevent double slashes in URLs

Strip trailing slashes from adminBasepath in AdminController window context
and wouter Router base to prevent paths like /admin//data when users
configure adminBasepath with a trailing slash.

Amp-Thread-ID: https://ampcode.com/threads/T-019ca537-4cc4-7174-bf9a-5325d782f097
Co-authored-by: Amp <amp@ampcode.com>

app/src/modules/server/AdminController.tsx

@@ -123,7 +123,7 @@ export class AdminController extends Controller {
                const obj: AdminBkndWindowContext = {
                   user: c.get("auth")?.user,
                   logout_route: authRoutes.logout,
-                  admin_basepath: this.options.adminBasepath,
+                  admin_basepath: this.options.adminBasepath.replace(/\/+$/, ""),
                   theme: this.options.theme,
                   logo_return_path: this.options.logoReturnPath,
                };