fix: preserve custom geofence names when toggling predefined areas

[hokiepokedad2]

Summary

• Root cause: syncSelectedFromAreas() in the Areas page was overwriting selectedAreas with only predefined area names on every checkbox toggle, silently dropping custom geofence kojiName entries
• Fix: Preserve custom geofence names (names not in the predefined areas list) across checkbox toggles, matching the preservation logic already used in saveAreas()
• Scope: Single method change in area-list.component.ts — the same pattern saveAreas() already uses at lines 264-270

Bug scenario (before fix)

1. User has custom geofence "my_park" active alongside predefined area "downtown"
2. User navigates to Areas page, toggles any predefined area checkbox
3. syncSelectedFromAreas() fires, overwrites selectedAreas to only predefined names — "my_park" is lost
4. User saves — backend receives only predefined areas, custom geofence is deactivated
5. User must navigate back to Geofences page to re-activate

After fix

Custom geofence names are preserved through checkbox toggles and included in the save payload.

Closes #109

Test plan

• Create a custom geofence and activate it for the current profile
• Navigate to the Areas page — verify custom geofence appears in the selected chips
• Toggle predefined area checkboxes on/off — verify custom geofence chip remains
• Save areas — verify custom geofence is still active on the Geofences page
• Clear all areas — verify custom geofence names are preserved (managed via Geofences page)
• Cancel changes — verify state reverts correctly including custom geofence names