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.

app.go

@@ -5648,6 +5648,7 @@ var bulkFieldColumns = map[string]string{
 	"sat_name":  "sat_name",
 	"sat_mode":  "sat_mode",
 	// Contacted station location + activation refs / SIG
+	"grid":     "grid",
 	"state":    "state",
 	"cnty":     "cnty",
 	"pota_ref": "pota_ref",