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/package.json

@@ -7,7 +7,9 @@
     "dev": "vite",
     "build": "vite build",
     "lint": "eslint .",
-    "preview": "vite preview"
+    "preview": "vite preview",
+    "test": "vitest run",
+    "test:watch": "vitest"
   },
   "dependencies": {
     "react": "^18.2.0",
@@ -23,6 +25,9 @@
   },
   "devDependencies": {
     "@eslint/js": "^9.30.1",
+    "@testing-library/jest-dom": "^6.4.0",
+    "@testing-library/react": "^15.0.0",
+    "@testing-library/user-event": "^14.5.0",
     "@types/react": "^18.2.62",
     "@types/react-dom": "^18.2.18",
     "@vitejs/plugin-react": "^4.6.0",
@@ -30,6 +35,8 @@
     "eslint-plugin-react-hooks": "^5.2.0",
     "eslint-plugin-react-refresh": "^0.4.20",
     "globals": "^16.3.0",
-    "vite": "^7.0.4"
+    "jsdom": "^24.0.0",
+    "vite": "^7.0.4",
+    "vitest": "^1.4.0"
   }
 }