feat(inspect): add & chart foreign metrics another OAP wrote to shared storage (#73)

Metrics Inspect can now add metrics the connected OAP doesn't define — ones
another OAP (an older version, or a different distribution that doesn't carry
the analysis rule) wrote into the shared storage — and chart their values.

A new "Foreign metric" tab in the + add metric drawer takes the metric name,
scope, and the storage value column + type (read from the catalog of the OAP
that does define the metric). Stage several with "+ add to list" and bulk-add
them, board-cap aware, through the same counting footer as the catalog tab.
Each widget enumerates the metric's entities and plots the value series via
OAP's new POST /inspect/values (the admin-port counterpart to execExpression,
which can't evaluate a metric the OAP has no local model for). Foreign widgets
carry a FOREIGN pill, behave like any other widget (entity nav, chart-cycle,
multi-select), and persist their selection + custom entities across reloads.

Absent-endpoint and upstream errors are reported honestly: a 404 on
/inspect/values says the OAP build is too old rather than "inspect disabled",
and OAP's own {error} text (a wrong value column, a metric that is actually
defined locally, an unsupported shape) now reaches the operator instead of a
bare HTTP 4xx.