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/__test__/ui/client/utils/AppReduced.spec.ts

@@ -203,6 +203,63 @@ describe("AppReduced", () => {
       });
    });
 
+   describe("withBasePath - double slash fix (admin_basepath with trailing slash)", () => {
+      it("should not produce double slashes when admin_basepath has trailing slash", () => {
+         const options: BkndAdminProps["config"] = {
+            basepath: "/",
+            admin_basepath: "/admin/",
+            logo_return_path: "/",
+         };
+
+         appReduced = new AppReduced(mockAppJson, options);
+         const result = appReduced.withBasePath("/data");
+
+         expect(result).toBe("/admin/data");
+         expect(result).not.toContain("//");
+      });
+
+      it("should work correctly when admin_basepath has no trailing slash", () => {
+         const options: BkndAdminProps["config"] = {
+            basepath: "/",
+            admin_basepath: "/admin",
+            logo_return_path: "/",
+         };
+
+         appReduced = new AppReduced(mockAppJson, options);
+         const result = appReduced.withBasePath("/data");
+
+         expect(result).toBe("/admin/data");
+      });
+
+      it("should handle absolute paths with admin_basepath trailing slash", () => {
+         const options: BkndAdminProps["config"] = {
+            basepath: "/",
+            admin_basepath: "/admin/",
+            logo_return_path: "/",
+         };
+
+         appReduced = new AppReduced(mockAppJson, options);
+         const result = appReduced.getAbsolutePath("data");
+
+         expect(result).toBe("~/admin/data");
+         expect(result).not.toContain("//");
+      });
+
+      it("should handle settings path with admin_basepath trailing slash", () => {
+         const options: BkndAdminProps["config"] = {
+            basepath: "/",
+            admin_basepath: "/admin/",
+            logo_return_path: "/",
+         };
+
+         appReduced = new AppReduced(mockAppJson, options);
+         const result = appReduced.getSettingsPath(["general"]);
+
+         expect(result).toBe("~/admin/settings/general");
+         expect(result).not.toContain("//");
+      });
+   });
+
    describe("edge cases", () => {
       it("should handle undefined basepath", () => {
          const options: BkndAdminProps["config"] = {

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,
                };

app/src/ui/routes/index.tsx

@@ -33,7 +33,7 @@ export function Routes({
 }) {
    const { theme } = useTheme();
    const ctx = useBkndWindowContext();
-   const actualBasePath = basePath || ctx.admin_basepath;
+   const actualBasePath = (basePath || ctx.admin_basepath).replace(/\/+$/, "");
 
    return (
       <div id="bknd-admin" className={theme + " antialiased"}>