Fix explorer back and session usage selection

PLAN.md

@@ -608,6 +608,14 @@ session now names its meter (`usageProvider`, from
 two lists cannot disagree) and `GET /usage` is matched on machine *and*
 provider. `None` is a session that meters nothing, and the phone draws
 nothing at all for it — not a zero, and not "unknown".
+The session's usage dialog applies the same machine-and-provider match and
+shows every billing pool for that provider; it does not turn opening one
+session into a comparison with the other providers on that machine.
+For a provider with several pools, the compact bar selects the pool named by
+the session's model (including Luna's `gpt-reserve` name), falling back to the
+provider's generic pool, and shows the shortest cycle that pool actually
+reports. A weekly-only pool therefore gets a weekly bar; it is never relabeled
+as five-hour merely because the provider called it `primary`.
 
 `DriverKind::Echo` names a meter of its own that exists only when a test has
 asked for one: `/usage` in an echo session sets an invented answer