Fix typecheck in template cross-domain test (#1628)

## What

Fixes a TypeScript error in `@hexclave/template` that was breaking `pnpm
typecheck` repo-wide:

```
client-app-impl.cross-domain.test.ts(450,101): error TS2345:
Argument of type '(options: { url: string | URL; }) => Promise<void>'
is not assignable to parameter of type '(...args: unknown[]) => unknown'.
```

## Why

The `vi.spyOn(...).mockImplementation(...)` callback typed its parameter
directly as `{ url: string | URL }`, but `mockImplementation` expects
`(...args: unknown[]) => unknown`. `unknown` isn't assignable to `{ url
}`, so the build failed.

## How

Accept variadic `unknown[]` args and cast `args[0]` to the expected
shape — matching the spy's actual signature while preserving the test's
behavior.

`pnpm --filter=@hexclave/template typecheck` now passes.

<!-- This is an auto-generated description by cubic. -->
---
## Summary by cubic
Fix TypeScript typecheck failure in `@hexclave/template` cross-domain
auth test by updating the `vi.spyOn(...)._redirectTo.mockImplementation`
to accept `(...args: unknown[])` and casting `args[0]` to `{ url: string
| URL }`. This aligns the mock with its expected signature and restores
`pnpm typecheck`.

<sup>Written for commit 4a1787ee66.
Summary will update on new commits.</sup>

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packages/template/src/lib/hexclave-app/apps/implementations/client-app-impl.cross-domain.test.ts

@@ -447,7 +447,8 @@ describe("StackClientApp cross-domain auth", () => {
     }));
     let currentHref = callbackUrl.toString();
     let redirectedUrl = "";
-    const redirectSpy = vi.spyOn(StackClientApp.prototype as any, "_redirectTo").mockImplementation(async (options: { url: string | URL }) => {
+    const redirectSpy = vi.spyOn(StackClientApp.prototype as any, "_redirectTo").mockImplementation(async (...args: unknown[]) => {
+      const options = args[0] as { url: string | URL };
       redirectedUrl = options.url.toString();
     });