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/api/dist/index.html

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     <meta name="theme-color" content="#0F1E3D" />
     <title>PilotVault · API Server</title>
-    <script type="module" crossorigin src="/assets/index-BPK80y5V.js"></script>
+    <script type="module" crossorigin src="/assets/index--4fMszzi.js"></script>
     <link rel="stylesheet" crossorigin href="/assets/index-CVh8EDzq.css">
   </head>
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