Add OpenWeather plugin with full Web App settings cascade

Builtin API Server connector for the OpenWeather API (current weather +
5-day forecast for a point, API-key auth, stdlib-only), surfaced to end
users through the same three-layer settings cascade as OpenSky/WebDAV.

- New builtin plugin (internal/plugins/builtin/openweather) with
  health probe, weather.current/forecast.5day capabilities, and tests.
- resolveOpenWeather cascade (global -> org -> user) with per-field
  independent resolution and API-key masking; GET/PUT/health endpoints.
- Web App BFF relays, api.js client, and a Settings card under
  APIs - External (scope switch, enable toggles, test connection).
- Resolver unit tests + rebuilt embedded frontend.

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

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

@@ -7,5 +7,6 @@ import (
 	_ "pilotvault/apiserver/internal/plugins/builtin/filetransfer"
 	_ "pilotvault/apiserver/internal/plugins/builtin/localstorage"
 	_ "pilotvault/apiserver/internal/plugins/builtin/opensky"
+	_ "pilotvault/apiserver/internal/plugins/builtin/openweather"
 	_ "pilotvault/apiserver/internal/plugins/builtin/webdav"
 )