wicket/wicketd: show how old SP data is

At the moment, on the dogfood rack's switch 1/sled 16, wicket says:

But this data is actually outdated and has been for a while. In the switch zone, /var/svc/log/oxide-wicketd:default.log is filled with messages of the form:

18:39:26.400Z WARN wicketd (wicketd MgsManager): Failed to get state for SP
    err = Error Response: status: 503 Service Unavailable; headers: {"content-type": "application/json", "x-request-id": "32a8fd38-c9f2-4142-a550-36351bece7ce", "content-length": "224", "date": "Wed, 07 Aug 2024 18:39:26 GMT"}; value: Error { error_code: Some("SpCommunicationFailed"), message: "error communicating with SP SpIdentifier { typ: Power, slot: 0 }: RPC call failed (gave up after 5 attempts)", request_id: "32a8fd38-c9f2-4142-a550-36351bece7ce" }
    file = wicketd/src/mgs/inventory.rs:306
    sp = SpIdentifier { slot: 0, type_: Power }

The fact that the data is shown, especially in green, makes it feel fresh and current even though it's been stale for a while. We should consider:

• showing the age of SP data
• making it yellow if it's too stale

cc @rmustacc, @andrewjstone, @jgallagher

Start with the failed SP-state handling at wicketd/src/mgs/inventory.rs:306 and compare it with the data shown for the dogfood rack's switch 1/sled 16. Trace how that state reaches wicket's display, then define and implement how its age is shown and when stale data should appear yellow. Done means outdated SP data is visibly distinguished from fresh data.