Panic in `ClientRequest::new` when using `base_path`

In one of my currently-migrating-projects, the first server function call panics. Here is what I managed to debug:

- The visible effect is that [this `unwrap`](https://github.com/DioxusLabs/dioxus/blob/v0.7.1/packages/fullstack/src/client.rs#L51) panics on `url::ParseError::RelativeUrlWithoutBase`.
- This happens because it tries to parse `"/{base_path}/api/{function}"` to `url::Url` (where `{base_path}` is the one defined in `Dioxus.toml`).
- Which in turn happens because `get_server_url` returned `"/{base_path}"` (which, not being empty, was not replaced with `"http://this.is.not.a.real.url:9000"`)
- … because at [the previous set point](https://github.com/DioxusLabs/dioxus/blob/v0.7.1/packages/dioxus/src/launch.rs#L326), `get_server_url()` returned an empty string, which got concatenated to the base path.

~~_(Looks like fixing this problem is not enough for my setup to work: The base path is missing when [generating the actual relative URL](https://github.com/DioxusLabs/dioxus/blob/v0.7.1/packages/fullstack/src/client.rs#L156) later.)_~~

**Environment:**

- Dioxus version: 0.7.1
- Rust version: 1.91.1
- OS info: NixOS 25.05
- App platform: web

Start with packages/fullstack/src/client.rs at the unwrap around line 51 and the relative URL generation around line 156, then inspect packages/dioxus/src/launch.rs around line 326. Reproduce the first server-function call with a Dioxus.toml base_path and trace the server URL and generated relative URL. Done means the call no longer panics and the base path is preserved in the resulting request URL.

Written by the indexing model from the issue text.