Add a user-defined custom-metric store alongside the built-in measurements
on the Measurements surface. A user defines an arbitrary metric (name, unit,
optional target window, decimals, description), logs values, and charts them.
The store is a separate generic catalog + values pair, isolated from the
closed measurement-type system: not synced, not in FHIR, not in insights —
log and chart only. Names and units are plaintext, mirroring the biomarker
catalog. The inline list lives beneath the measurement list (no new nav
entry); tapping a metric opens a detail page with the trend chart, target
band, stat strip, and an editable value history.
Migration 0224 adds custom_metrics and custom_metric_entries.
