feat: add Brave Search as a web search provider

[AndrewDongminYoo]

What

Adds Brave Search API as a web search provider alongside Tavily, SerpApi, SearXNG, Ollama, and Firecrawl.

Changes

• src/stores/wiki-store.ts — adds "brave" to SearchProvider union type
• src/lib/web-search.ts — implements braveSearch(), routes the switch case, updates the API-key validation error to mention Brave
• src/components/settings/sections/web-search-section.tsx — adds Brave Search entry to the provider list in Settings
• src/lib/web-search.test.ts — three new tests: happy path with header/result normalization, 20-result count cap, 401 auth error surface

Implementation details

• Auth: X-Subscription-Token: <api_key> (Brave's published scheme, distinct from Authorization: Bearer)
• Endpoint: GET https://api.search.brave.com/res/v1/web/search
• Result cap: count is clamped to [1, 20] — Brave silently caps higher values server-side; clamping client-side keeps the URL honest and avoids surprising callers
• Response mapping: data.web.results[] → { title, url, snippet: description, source: hostnameFromUrl(url) } — same shape as existing providers
• Error handling: dedicated 401/403 message ("check your subscription token"), network error via isFetchNetworkError(), generic HTTP error with status + body
• No new dependencies

Test plan

• TypeScript: npx tsc --noEmit passes
• Unit tests: all 19 passing (3 new Brave tests + existing suite intact)
• Manual: Settings → Web Search → Brave Search appears in provider list
• Manual: configure Brave API key → run Deep Research → results render correctly