feat(search): add Exa AI as web search provider

[6tizer]

Summary

• Add Exa as a web search provider option for Deep Research
• Exa returns full page content via its contents.text API, giving the LLM synthesis stage richer material than short snippets from other providers

Changes

• src/stores/wiki-store.ts — Add "exa" to SearchProvider type union
• src/lib/web-search.ts — Implement exaSearch() function using x-api-key header auth, contents: { text: true } request; add switch case and API key validation
• src/components/settings/sections/web-search-section.tsx — Add Exa entry to SEARCH_PROVIDERS array (API key only, no additional config needed)
• src/lib/web-search.test.ts — Update validation error message assertion

Technical Details

• Auth: x-api-key header (not in request body, not Bearer token)
• Endpoint: POST https://api.exa.ai/search
• Response mapping: r.text || r.highlights?.[0] || r.summary || "" → snippet fallback chain
• Error handling: Dedicated 401 auth error, network error via isFetchNetworkError(), generic HTTP error with status code

Test plan

• TypeScript compilation passes with 0 errors
• web-search.test.ts — 12/12 tests pass
• Manual: Settings → Web Search → Exa appears as provider option
• Manual: Configure Exa API key → run Deep Research → verify results with full page content
• Verify no regressions in existing Tavily/SerpApi/SearXNG/Ollama providers

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