Chat: OpenRouter as a chat provider on Windows and Linux (chat only)

[4rchila]

What

OpenRouter as a second chat provider on Windows and Linux, next to the Claude API. On macOS the chat already supports Google AI and OpenAI; the Tauri build only had Anthropic. OpenRouter gives access to many models through one key, including free ones, which makes the chat usable without an Anthropic API key.

How

• openai_compat.rs: a small OpenAI-compatible /chat/completions client, built like claude.rs:
  • multi-turn history on the Rust side; only successful turns enter it;
  • the key lives in the keychain (openrouter-api-key added to KNOWN_KEYS), never on disk and never in the front end;
  • chat only: no tools are sent, so the model can answer but never act on the machine;
  • images (data URL) and text files ride along with the first message, like the Claude chat; PDFs are not sent (no portable document part in the OpenAI format);
  • readable errors for 401 (bad key), 402 (no credits) and 429 (rate limit, common on free models).
• openrouter_models command: OpenRouter's public catalogue (no key needed), text-output models, free ones first.
• Settings → Chat provider: choose Claude API or OpenRouter, save/remove the key, pick a model with a "Free only" filter.
• chat_send dispatches on settings.chatProvider (default "anthropic", so nothing changes for existing users); chat_reset clears both histories.

New settings fields have serde defaults, so older settings.json files still load. No new dependency (reqwest and serde_json were already there). Network calls only go to openrouter.ai once the user picks OpenRouter (the model list is fetched when Settings opens).

Testing

• Fedora 44 / KDE Plasma: saved a key, loaded the model list (free filter on and off) and chatted with a free model.
• tsc --noEmit, vite build and cargo check --release pass with no new warnings.
• Not tested on Windows (no machine), but the code is platform-independent.

Screenshots

Settings → Chat provider (taken from my local build; the duplicated "(free) (free)" label and the "KWallet / Secret Service" wording visible there are already fixed in this PR: OpenRouter's free model names already contain "(free)", and the hint now just says "system keychain"):