dioxus-fullstack-core: silently broken SSR hydration when both web and server features are enabled

## Summary

When `dioxus-fullstack-core` is compiled with both `web` and `server` features enabled simultaneously, SSR hydration silently breaks. The `serialize_context()` function uses `#[cfg(feature = "web")]` / `#[cfg(not(feature = "web"))]` to branch between client
and server code paths. When both features are on, the `web` path always wins — even on the server — causing `use_loader` data to never be serialized into the hydration blob.

## Steps to Reproduce

1. Create a Dioxus fullstack app with `use_loader` + `SuspenseBoundary`
2. Enable both `web` and `server` features on `dioxus-fullstack-core` (e.g. via Cargo feature unification)
3. Run `dx serve`
4. Open any page with `use_loader(...)?`

## Expected Behavior

SSR renders the page and sends hydration data to the client. Client hydrates without errors.

## Actual Behavior

- `window.initial_dioxus_hydration_data` contains only the error entry (4 bytes) — zero loader data
- Client receives no serialized server function results
- Client's virtual DOM has fewer nodes than the server HTML
- Hydration crashes: `Cannot read properties of undefined (reading 'toString')` in `hydrate_node`

## Root Cause

https://github.com/DioxusLabs/dioxus/blob/e5b9c39f4e53ce86c0b1af2fb077913cf28f9365/packages/fullstack-core/src/transport.rs#L162-L180

When `web` is enabled on the server, the `else` branch returns a throwaway `HydrationContext` (the thread-local `CONTEXT` is never set on the server). This context is never `provide_context()`-ed into the scope, so the SSR tree walk in `serialize_server_data`
finds no hydration data in any scope.

## Proposed Fix

Add a compile-time error in `dioxus-fullstack` when both features are enabled:

```rust
#[cfg(all(feature = "web", feature = "server"))]
compile_error!(
"The `web` and `server` features of `dioxus-fullstack` are mutually exclusive."
);
```

**Environment:**

- Dioxus version: v0.7.4
- Rust version: 1.86.0
- OS info: macOS 15.4 (aarch64)
- App platform: web (fullstack SSR)

**Questionnaire**

I would like to fix and I have a solution.