Update generated templates to match the current GPUI startup API

[andyhennie]

I hit this while trying a fresh scaffold locally.

The generated app still uses Application::new().run(...), but current GPUI examples have moved to gpui_platform::application(). With a newly generated project, the first cargo run fails because Application::new is no longer available.

This updates the generated templates to use gpui_platform::application() and adds the matching dependency in both the single-crate and workspace templates.

What I verified locally:

• generated a default app with the patched CLI and ran cargo check
• generated a workspace app with the patched CLI and ran cargo check

Repro before this change:

cargo install create-gpui-app
create-gpui-app --name my-app
cd my-app
cargo run

[andyhennie]

Would appreciate a review on whether this is the right place and shape for the fix. I hit this from a fresh scaffold locally and kept the patch as small as I could.