Show when the data was last updated #115

Blocked by: #107

Problem

If the events refresh or an upload failed, the rates may be out of date and the manager has no way to know.

Goal

The manager can always see when events, weather, hotel data and the model were last updated, and is warned when something is stale.

Scope

• A small indicator in the signed-in layout (e.g. "Updated 2 hours ago").
• Opening it shows the last successful events refresh, weather refresh, upload, training and rate update.
• A clear warning when any of them is stale or the latest attempt failed, with what to do (e.g. "Upload today's exports").

Design

• No design exists yet. Design the indicator and its expanded view before building.
• Keep it quiet when everything is fresh; only draw attention when something is stale.

Acceptance criteria

• The design is agreed before building.
• The indicator shows the most recent update time (against mocks until the backend is ready).
• Stale or failed items show a warning with a next step.