Add bounding-box region picker to the API Server plugin panel

Give the superadmin Plugins panel the same OpenSky "Default bounding
box" experience as the Web App: a grouped preset dropdown (World /
Continents / Countries) plus a Custom option for manual
lamin,lomin,lamax,lomax entry.

Introduce a "bbox" config-field type (rendering hint only; unknown
types still degrade to a text input) and mark the OpenSky bbox field
with it. Values that match a preset render as the region name.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

API Server/internal/plugins/builtin/opensky/opensky.go

@@ -78,7 +78,7 @@ func (p *Plugin) Descriptor() plugins.Descriptor {
 				Help: "Global account tier. Leave it unset to let each organization or user pick their own plan; set a value only to force one plan for everyone. Determines the daily credit allowance shown next to remaining credits."},
 			{Key: "clientId", Label: "OAuth2 client ID", Type: "text", Help: "Optional — leave blank for anonymous access (lower rate limits)."},
 			{Key: "clientSecret", Label: "OAuth2 client secret", Type: "password", Secret: true, Help: "Paired with the client ID for authenticated access."},
-			{Key: "bbox", Label: "Default bounding box", Type: "text", Default: defaultBBox, Help: "lamin,lomin,lamax,lomax — used by the health probe and states.bbox."},
+			{Key: "bbox", Label: "Default bounding box", Type: "bbox", Default: defaultBBox, Help: "Pick a region or choose Custom to enter lamin,lomin,lamax,lomax — used by the health probe and states.bbox."},
 			{Key: "allowAnonymous", Label: "Anonymous access", Type: "select", Default: "true",
 				Options: []plugins.SelectOption{
 					{Value: "true", Label: "Enabled — allow use without credentials"},