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>

Makefile

@@ -242,6 +242,11 @@ test-integration:
 	@echo "Running full integration tests (requires running PIC stack)..."
 	PIC_HOST=$${PIC_HOST:-localhost} python3 -m pytest tests/integration/ -v
 
+test-webui:
+	@echo "Running webui unit tests (requires node; builds a disposable container)..."
+	docker run --rm -v "$(PWD)/webui:/app" -w /app node:18-alpine \
+		sh -c "npm install --silent && npm test"
+
 test-integration-readonly:
 	@echo "Running read-only integration tests (no peer creation)..."
 	PIC_HOST=$${PIC_HOST:-localhost} python3 -m pytest tests/integration/test_live_api.py tests/integration/test_webui.py -v