feat(app2): introduce Effect runtime (#4538)

Previously we had no way to provision Effect requirements in an app-wide
manner. This changeset introduces a
[`ManagedRuntime`](https://effect.website/docs/runtime/#managedruntime)
as a module singleton.

The runtime is guaranteed to exist upon page load by virtue of
SvelteKit's
[`init`](https://svelte.dev/docs/kit/hooks#Shared-hooks-init) hook.

Initial integration targets the following features:
- Integration of a [custom Effect
`Logger`](https://effect.website/docs/observability/logging/#custom-loggers)
for Datadog interoperability.
- Provisioning of runtime to the in-house Effect "runes" which handle
automatic resource cleanup and interruption (see #4444).
- Minimal dependency inversion exchanging a "Live" layer with a "Test"
layer for integration testing when running via `vitest`.