Add app.add_instrument() feature to FastHTML

Per @jph00 would be nice for a built-in FastHTML control that does this and requires behavioural change of library:

```python
app = fast_app()
app.add_instrument() # Adds Profile middleware to app, or raises import error if fh-pyinstrument not installed

@rt
@instrument # Only works if app.add_instrument called
def index(): return Titled('Hello')
```

Start by tracing the existing FastHTML app setup and decorator entry points to determine where app.add_instrument() and @instrument would integrate. Confirm the intended behavior: add Profile middleware, raise an import error when fh-pyinstrument is unavailable, and only enable @instrument after the app method is called.

Written by the indexing model from the issue text.