feat(plugin-chart-country-map): friendlier labels + auto-migrate from legacy plugin

Two small UX wins on top of the previous control-panel fixes:

1. Rename admin_level options to "World" / "Country" / "Aggregated
   regions" (from the technical "Countries (Admin 0)" / "Subdivisions
   (Admin 1)"). The control's own label becomes "Map view" with an
   explanatory description. Underlying form_data values stay 0 / 1 /
   aggregated so saved charts don't break.

2. Auto-migrate form_data when a user switches a saved legacy
   country_map chart's viz type to country_map_v2:
     - select_country: 'france'         → admin_level=1, country=FRA
     - select_country: 'france_overseas' → composite=france_overseas
     - select_country: 'turkey_regions'  → admin_level=aggregated,
                                            country=TUR, region_set=nuts_1
     - …same pattern for france_regions / italy_regions /
       philippines_regions and ~180 other legacy country files.

   The mapping lives in src/plugin/migrateFromLegacy.ts (covered by
   12 unit tests). Migration only fills fields the user hasn't already
   set on the new chart, so explicit edits win over inferred values.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>