feat: support self-hosted Firecrawl search

[iamvaleriofantozzi]

Summary

Enable self-hosted Firecrawl as a search provider alongside existing SearXNG.

Changes

• Configuration (stores/wiki-store.ts, web-search-section.tsx): added firecrawlUrl and optional apiKey fields to the search-provider configuration UI
• Provider (lib/web-search.ts): added SelfHostedFirecrawlSearch function that calls the self-hosted Firecrawl instance instead of the upstream anonymous-cloud endpoint. Removed upstream anonymous-cloud-only implementation
• i18n (en.json, zh.json): added self-hosted Firecrawl hint text
• Tests (lib/web-search.test.ts): updated tests to require firecrawlUrl; removed 10 upstream cloud-tests that no longer apply

Diff summary

6 files changed, 206 insertions(+), 276 deletions(-)