Use useCurrentApp hook in AppLoader instead of manual observable wiring

## Context

`AppLoader` in [`packages/dev-portal/src/AppLoader.tsx`](https://github.com/equinor/fusion-framework/blob/main/packages/dev-portal/src/AppLoader.tsx) currently observes the current application via:

```ts
const { value: currentApp } = useObservableState(
useMemo(() => fusion.modules.app.current$, [fusion.modules.app]),
);
```

## Task

Change this to use a `useCurrentApp` hook (once available) instead of manually wiring `useObservableState` + `useMemo` around `fusion.modules.app.current$`.

## Acceptance criteria

- [ ] `useCurrentApp` hook exists (or is confirmed to already exist) and is adopted here
- [ ] Behavior unchanged (still re-renders when the current app changes)

Start in packages/dev-portal/src/AppLoader.tsx and search the repository for an existing useCurrentApp hook. Confirm whether the hook exists or identify where it should come from, then verify that AppLoader still re-renders when fusion.modules.app.current$ changes. Done means AppLoader uses useCurrentApp and its behavior remains unchanged.

Written by the indexing model from the issue text.