Add label overview page

# Context

`frontend/src/routes/_layout/$productType/labels/$labelId.tsx` is only an outlet; there is no index route. Clicking a label from the list goes directly into a sub-section (edit / compliance / files) with no overview landing page summarizing the label's state.

Related: #680 (label review lifecycle redesign) — the overview page is the natural surface for whatever lifecycle representation results from that work.

# Outcome

Opening a label lands on an overview page that shows, at a glance, what the label is, where it stands in the workflow, and what action is expected next.

# Acceptance criteria

- A label detail route exists at the `$labelId` index
- The page shows the label's identity (product link if any, product type, creation/update timestamps, creator)
- The page surfaces the label's current lifecycle state (as defined by #680)
- The page surfaces a summary of extraction status and compliance results
- The page provides navigation to the existing sub-sections (edit, compliance, files)
- The page indicates what action, if any, is expected of the current user