feat(webui): internet sharing UI — exit-offer toggle + peer route-via selector

CellNetwork page (CellPanel):
- Internet Sharing section below service toggles
- Toggle: 'Offer my internet to <cell>' (calls PUT /api/cells/<n>/exit-offer)
- Read-only indicator: whether remote cell offers internet back
- Contextual hints explaining what each party needs to do next

Peers page:
- Fetches connected cells on mount
- Edit modal: Internet Exit dropdown (route-via) showing all connected cells
  with ✓ marker for cells that have offered internet
- Warning if selected cell hasn't offered internet yet
- On save, calls PUT /api/peers/<n>/route-via only when value changed
- Table badge shows 'via <cell>' for peers with active routing

api.js:
- cellLinkAPI.setExitOffer(cellName, offered)
- peerRegistryAPI.setRouteVia(peerName, viaCell)

Tests (vitest + @testing-library/react):
- 19 new frontend tests in src/__tests__/
  - CellNetworkInternetSharing.test.jsx (10 tests)
  - PeersRouteVia.test.jsx (9 tests)
- make test-webui target runs them via docker node:18-alpine

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

webui/vite.config.js

@@ -4,6 +4,11 @@ import react from '@vitejs/plugin-react'
 // https://vitejs.dev/config/
 export default defineConfig({
   plugins: [react()],
+  test: {
+    environment: 'jsdom',
+    globals: true,
+    setupFiles: ['./src/test-setup.js'],
+  },
   server: {
     port: 5173,
     proxy: {