Update Sentry example

[HazAT]

When setting up a fresh tanstack start app and chosing Sentry, it sets it up but when running dev locally first, it fails because no DSN is configured by default. This PR aims to smoothen the UX.

Before

pnpn run dev fails in a fresh project with this

After

and also added this warning banner + disabling the buttons on the example page:

[TkDodo]

we can do this directly in the useState initializer:

const [sentryConfigured] = useState(() => {
  // Check if Sentry DSN environment variable is set
  const hasDsn = !!import.meta.env.VITE_SENTRY_DSN
})

also, we’re checking for import.meta.env here, but the server instrumentation file does:

const sentryDsn = import.meta.env?.VITE_SENTRY_DSN ?? process.env.VITE_SENTRY_DSN