feat(bulk): My DXCC / My CQ zone / My ITU zone

The My-station group covered nineteen fields and not the three numeric ones.
They take their own integer path rather than the generic text setter: the
columns are nullable integers, and while SQLite would coerce '14' quietly, a
shared MySQL logbook would not — and empty must become NULL, never ''. Bounds
checked (DXCC <1000, CQ 1-40, ITU 1-90) so a slip cannot stamp zone 400 across
a thousand rows.

frontend/src/components/BulkEditModal.tsx

@@ -60,6 +60,9 @@ const FIELDS: FieldDef[] = [
   // No promoted column: written into extras_json (see qso.bulkEditableExtras).
   { id: 'owner_callsign',   label: 'bulk.fOwnerCallsign', group: 'My station', kind: 'text', upper: true },
   { id: 'my_grid',          label: 'bulk.fMyGrid',       group: 'My station', kind: 'text', upper: true },
+  { id: 'my_dxcc',          label: 'bulk.fMyDxcc',       group: 'My station', kind: 'text' },
+  { id: 'my_cq_zone',       label: 'bulk.fMyCqZone',     group: 'My station', kind: 'text' },
+  { id: 'my_itu_zone',      label: 'bulk.fMyItuZone',    group: 'My station', kind: 'text' },
   { id: 'my_antenna',       label: 'bulk.fMyAntenna',    group: 'My station', kind: 'text' },
   { id: 'my_rig',           label: 'bulk.fMyRig',        group: 'My station', kind: 'text' },
   { id: 'my_street',        label: 'bulk.fMyStreet',     group: 'My station', kind: 'text' },