feat(frontend): host les fenêtres plugin et l'API publique windows.open

Ajoute la vue window host qui charge le layout plugin dans une fenêtre OS
détachée, expose react/react-dom/jsx-runtime hébergés sous
frontend/public/plugin-host pour que le runtime SDK y résolve React, et
câble services.windows.open() côté runtime plugin (loader/services/registry).

npm test -- --run src/adapters/window.test.ts src/app/ViewWindow.test.tsx
  src/plugins/runtime/loader.test.ts src/plugins/runtime/services.test.ts : 39/39
npm run typecheck : OK
npm run build : OK

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

frontend/src/features/plugins/PluginRuntimeProvider.tsx

@@ -79,6 +79,7 @@ export function PluginRuntimeProvider({ children, value: injected }: PluginRunti
       terminal: gateways.terminal,
       agents: gateways.agent,
       system: gateways.system,
+      window: gateways.window,
       workState: gateways.workState,
       focusedProject: gateways.focusedProject,
       pluginWorkspace: gateways.pluginWorkspace,