Add region picker for OpenSky default bounding box

Replace the free-text bounding-box field in the Web App OpenSky
settings with a grouped preset picker (World / Continents / Countries)
plus a Custom option that keeps the manual lamin,lomin,lamax,lomax
entry. Saved values that match a preset render as the friendly region
name, including in the locked/inherited display.

Change the API Server default bounding box to Europe to match the new
"Europe" preset.

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

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

@@ -25,8 +25,10 @@ import (
 const (
 	apiBase  = "https://opensky-network.org/api"
 	tokenURL = "https://auth.opensky-network.org/auth/realms/opensky-network/protocol/openid-connect/token"
-	// Small default bounding box (Netherlands) keeps the health probe cheap.
-	defaultBBox = "50.5,3.2,53.7,7.3" // lamin,lomin,lamax,lomax
+	// Default bounding box covers Europe; matches the "Europe" preset offered in
+	// the Web App bounding-box picker. Users can narrow it (e.g. a single country)
+	// to reduce per-query credit cost.
+	defaultBBox = "34,-25,72,45" // lamin,lomin,lamax,lomax (Europe)
 )
 
 func init() {