feat: bulk-edit the contacted station's gridsquare

An import that drops the locator leaves it missing on a whole batch, and fixing
that one QSO at a time is exactly what bulk edit exists to avoid.

Added on all three sides in lockstep — the dialog field, the field-to-column map,
and the repository whitelist — because they are separate lists and offering a
field the repository refuses fails only at Apply, after the operator has selected
the QSOs. The existing contract test covers it.

The callsign and RST stay excluded, as before: bulk-setting those corrupts a log.
The locator does not carry that risk — it identifies a place, not a station, and
a wrong value is simply overwritten again.

frontend/src/components/BulkEditModal.tsx

@@ -82,6 +82,7 @@ const FIELDS: FieldDef[] = [
   { id: 'sat_name',  label: 'bulk.fSatName',  group: 'Propagation', kind: 'text', upper: true },
   { id: 'sat_mode',  label: 'bulk.fSatMode',  group: 'Propagation', kind: 'text', upper: true },
   // Contacted station (location / activation refs / SIG)
+  { id: 'grid',     label: 'bulk.fGrid',    group: 'Contacted station', kind: 'text', upper: true },
   { id: 'state',    label: 'bulk.fState',   group: 'Contacted station', kind: 'text', upper: true },
   { id: 'cnty',     label: 'bulk.fCnty',    group: 'Contacted station', kind: 'text' },
   { id: 'pota_ref', label: 'bulk.fPotaRef', group: 'Contacted station', kind: 'text', upper: true },