feat(claude): support custom ANTHROPIC_BASE_URL and proxy endpoints

[Totopo27]

Summary

Adds support for configuring a custom Anthropic API base URL via the ANTHROPIC_BASE_URL environment variable or directly in the Claude Settings view, enabling users to route requests through self-hosted proxies, LiteLLM, Ollama proxies, or enterprise gateways.

Closes #26

Motivation & Root Cause

The Anthropic messages endpoint was hardcoded to https://api.anthropic.com/v1/messages across both the Windows backend and macOS service. Users running third-party proxies, local model gateways, or custom network setups could not direct Mochi's chat queries to their custom endpoints.

Key Changes

• windows/src-tauri/src/claude.rs:
  • Added resolve_endpoint(custom) helper that normalizes custom base URLs (appending /v1/messages if omitted, respecting explicit /messages or /v1 paths, and falling back to ANTHROPIC_BASE_URL then the default Anthropic endpoint).
  • Updated send and call to dispatch to the resolved endpoint.
  • Added unit test covering standard proxy URL formats.
• windows/src-tauri/src/settings.rs & windows/src-tauri/src/lib.rs:
  • Added optional anthropic_base_url to Settings.
  • Forwarded custom base URL from settings into claude::send.
• windows/src/core/state.ts & windows/src/settings/main.ts:
  • Added "Base URL" configuration input row under the Claude settings section.
• NotchBuddy/Sources/App/ClaudeService.swift:
  • Updated endpoint and fetchModels on macOS to inspect ANTHROPIC_BASE_URL environment variable when present.

Verification

• cargo test claude passed (including endpoint_resolution_handles_various_url_formats).
• npx tsc --noEmit clean on the frontend.

[Totopo27]

I created a more surgical, env-only alternative in #206 to resolve #180. If UI settings aren't desired yet, feel free to close this in favor of #206.

[Totopo27]

Closing in favor of the upstream solution implemented in #307 / Windows v0.2.0 via COUCOU_ANTHROPIC_BASE_URL. Thanks @Louis-CFM!