feat: add OpenRouter provider support

[Rish-it]

Adds OpenRouter as an LLM provider, enabling unified access to multiple models via a single API key.

Changes:

• llm.ts: Add model factories for openai/, anthropic/, google/ prefixes
• ModelSelector.tsx: Add OpenRouter with GPT-5.2, Claude Opus 4.5, Gemini 3 Pro
• env.ts: Add openrouter to provider config
• env.example & README.md: Document usage

Closes #26