feat: add configurable API base URL (fixes #7, closes #9)

[shivamtiwari3]

Summary

• Fixes Request to add a custom baseUrl #7 (request to add a custom base URL)
• Closes Are there plans to support OpenAI endpoints #9 (support for OpenAI-compatible endpoints)

Adds a new API Base URL field to the Settings page, letting users point the app at any proxy or compatible endpoint instead of the hardcoded Anthropic URL.

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Problem

The API endpoint was hardcoded to https://api.anthropic.com/v1/messages in config.ts and used directly in agent-worker.ts. Users who run a local proxy, a corporate gateway, or want to experiment with OpenAI-compatible endpoints that mirror the Anthropic messages API had no way to override it.

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Solution

• src/config.ts — added BASE_URL key to CONFIG_KEYS.
• src/types.ts — added optional baseUrl?: string to InvokePayload and CompactPayload.
• src/orchestrator.ts — new private baseUrl field loaded from IndexedDB on init(); new getBaseUrl() / setBaseUrl() methods; field forwarded in both invoke and compact worker messages.
• src/agent-worker.ts — handleInvoke and handleCompact resolve apiUrl = baseUrl || ANTHROPIC_API_URL, so the default is completely unchanged when the field is blank.
• src/components/settings/SettingsPage.tsx — new API Base URL card between the API Key and Model sections with a URL input, Save button, and confirmation feedback.

Leaving the field empty keeps existing behaviour — no breaking change.

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Testing

• npm run typecheck — passes with zero errors.
• Manually tested: blank field uses default Anthropic endpoint as before; setting a custom URL routes requests to that endpoint.

Checklist

• Fixes the reported issue
• Existing behaviour unchanged when field is left blank (no regressions)
• TypeScript type-checks cleanly (npm run typecheck)
• Code style matches project conventions (single quotes, no extra abstractions)