dashboard polish: integer values, map panel, stacked deploys

[kualta]

Summary

• Integer Y-axes: panels now set `config.decimals: 0` so counts render as `240` not `240.00`.
• Choropleth map panel: `type = "maps"` is now supported. iron auto-routes `x` -> `fields.name` and `y` -> `fields.value_for_maps`, and sets a default `base_map` + `map_view` (world view).
• Stacked bars: the Deploys chart now uses `type = "stacked"` with `breakdown = "app"`.
• Visitors by country chart added (full-width, fleet tab). Country codes derived from `device_locale` for now; GeoIP-based enrichment is a follow-up.

Test plan

• cargo fmt, clippy -D warnings, test
• iron deploy observe -> "Dashboard synced (8 charts)"
• map panel JSON has type=maps, fields.name, fields.value_for_maps, config.map_view, config.base_map
• SQL returns GB = 240 from _rumdata