feat: add OpenRouter as LLM provider

[MinimaxLanbo]

Summary

• Add OpenRouter as a new LLM provider, using OpenAI-compatible protocol
• Users can now access MiniMax and other models via OpenRouter's unified API endpoint
• Preset models include MiniMax, Claude, GPT, DeepSeek, Gemini series

Closes #22

Test plan

• Unit tests pass (pnpm test — 107 passed)
• Lint pass (pnpm run lint)
• Build pass (pnpm build)
• Manual: select OpenRouter provider, enter API key, send message

🤖 Generated with Claude Code

[claude]

Code Review: PR #26 - Add OpenRouter as LLM Provider

Summary

Adds OpenRouter as a new LLM provider option with MiniMax M2.5 as the default model.

Changes Made

• Added openrouter to LLMProvider type union
• Added OpenRouter provider config with 9 models (MiniMax, Claude, GPT, DeepSeek, Gemini)
• Added test case for getDefaultProviderConfig('openrouter')
• Added dropdown option in ChatPanel settings

Issues Found

1. Model ID Format Inconsistency (Low)
The OpenRouter models use a different ID format (provider/model-id) compared to direct providers:

• minimax/MiniMax-M2.5 vs MiniMax-M2.5
• anthropic/claude-sonnet-4-6 vs claude-sonnet-4-6

This is intentional for OpenRouter's API but worth noting. The test passes correctly.

2. Test Coverage (Good)
The test verifies:

• Provider name
• Base URL
• Default model

All assertions match the config values correctly.

Verdict: ✅ Approve

The implementation is correct and follows existing patterns:

• Type is properly extended
• Config follows the same structure as other providers
• Test validates defaults match
• UI dropdown option added

No bugs, type safety issues, or security concerns found.