feat(rn): [P1] Web search settings

[liuyidi]

Summary

Web/Browser settings: search provider, API key, max results/timeout, Jina Reader for web_fetch.

WebUI reference

• webui/src/pages/settings/browser/WebSettings.tsx
• webui/src/pages/settings/browser/useBrowserSettings.ts

API

• …/web-search/update and related settings fields

RN landing

• New: src/app/settings/browser.tsx

Acceptance

• Can set search provider / key / max results
• Persist via settings API

Depends

• chore: Settings API thin wrapper