Create app.py instead of hello.py in uv init

[Gankra]

From the discussion, it seems like app.py maybe has one benefit (at least one tool seems to autodetect it) and main.py maybe has one downside (__main__ is already a thing and people might think this is related or get confused). So on balance if we have to pick one, app.py seems a bit better.

Fixes #7782

[Gankra]

The only concern I have with this is it changes intro docs, so there will be an awkward period where people have older versions of uv that make hello.py but the guide says they make app.py. Dunno if we want to make the docs hedge for a bit or what.

[zanieb]

The only concern I have with this is it changes intro docs, so there will be an awkward period where people have older versions of uv that make hello.py but the guide says they make app.py. Dunno if we want to make the docs hedge for a bit or what.

I'd just go for it — you could add an admonition about older versions if you want though.

[zanieb]

Arguably this is sort of breaking, though I'm not sure we consider uv init to be a typical programmatic interface. @charliermarsh thoughts?

If we decide it's breaking, we'll just hold it until 0.6.0 (for which there is a milestone to track other breaking changes).

[charliermarsh]

It probably makes sense to defer this to v0.6. We could do a v0.6 in the near future though.

[zanieb]

Sorry for giving you an unmergable task!