[FEATURE] Add path to jupy HTMX

[Isaac-Flath]

Is your feature request related to a problem? Please describe.
the HTMX function to run app in a notebook does not allow for passing a path (though Hamel has a clever hack of passing port=8000/route). To make this cleaner, we should add a path that you can pass to HTMX to load a particular route in the iframe.

Describe the solution you'd like
HTMX() -> Loads / route in iframe
HTMX(myroute) -> loads /myroute` in iframe

Example code
Provide an example of how you imagine the feature working:

HTMX('myroute')

Problem solved
It would make using fastHTML apps in jupyter notebooks cleaner.

Additional context

Confirmation
Please confirm the following:

• I have checked the existing issues and pull requests to ensure this feature hasn't been requested before.
• I have read the project's documentation to ensure this feature doesn't already exist.