docs: add AI provider configuration guide (#100)

- Add docs/ai-providers.md with detailed setup instructions for all providers
- Update README.md, README_CN.md, README_JA.md with provider guide links
- Add model capability requirements note
- Simplify provider list in READMEs

Closes #79

docs/ai-providers.md

@@ -0,0 +1,141 @@
+# AI Provider Configuration
+
+This guide explains how to configure different AI model providers for next-ai-draw-io.
+
+## Quick Start
+
+1. Copy `.env.example` to `.env.local`
+2. Set your API key for your chosen provider
+3. Set `AI_MODEL` to your desired model
+4. Run `npm run dev`
+
+## Supported Providers
+
+### Google Gemini
+
+```bash
+GOOGLE_GENERATIVE_AI_API_KEY=your_api_key
+AI_MODEL=gemini-2.0-flash
+```
+
+Optional custom endpoint:
+
+```bash
+GOOGLE_BASE_URL=https://your-custom-endpoint
+```
+
+### OpenAI
+
+```bash
+OPENAI_API_KEY=your_api_key
+AI_MODEL=gpt-4o
+```
+
+Optional custom endpoint (for OpenAI-compatible services):
+
+```bash
+OPENAI_BASE_URL=https://your-custom-endpoint/v1
+```
+
+### Anthropic
+
+```bash
+ANTHROPIC_API_KEY=your_api_key
+AI_MODEL=claude-sonnet-4-5-20250514
+```
+
+Optional custom endpoint:
+
+```bash
+ANTHROPIC_BASE_URL=https://your-custom-endpoint
+```
+
+### DeepSeek
+
+```bash
+DEEPSEEK_API_KEY=your_api_key
+AI_MODEL=deepseek-chat
+```
+
+Optional custom endpoint:
+
+```bash
+DEEPSEEK_BASE_URL=https://your-custom-endpoint
+```
+
+### Azure OpenAI
+
+```bash
+AZURE_API_KEY=your_api_key
+AI_MODEL=your-deployment-name
+```
+
+Optional custom endpoint:
+
+```bash
+AZURE_BASE_URL=https://your-resource.openai.azure.com
+```
+
+### AWS Bedrock
+
+```bash
+AWS_REGION=us-west-2
+AWS_ACCESS_KEY_ID=your_access_key_id
+AWS_SECRET_ACCESS_KEY=your_secret_access_key
+AI_MODEL=anthropic.claude-sonnet-4-5-20250514-v1:0
+```
+
+Note: On AWS (Amplify, Lambda, EC2 with IAM role), credentials are automatically obtained from the IAM role.
+
+### OpenRouter
+
+```bash
+OPENROUTER_API_KEY=your_api_key
+AI_MODEL=anthropic/claude-sonnet-4
+```
+
+Optional custom endpoint:
+
+```bash
+OPENROUTER_BASE_URL=https://your-custom-endpoint
+```
+
+### Ollama (Local)
+
+```bash
+AI_PROVIDER=ollama
+AI_MODEL=llama3.2
+```
+
+Optional custom URL:
+
+```bash
+OLLAMA_BASE_URL=http://localhost:11434
+```
+
+## Auto-Detection
+
+If you only configure **one** provider's API key, the system will automatically detect and use that provider. No need to set `AI_PROVIDER`.
+
+If you configure **multiple** API keys, you must explicitly set `AI_PROVIDER`:
+
+```bash
+AI_PROVIDER=google  # or: openai, anthropic, deepseek, azure, bedrock, openrouter, ollama
+```
+
+## Model Capability Requirements
+
+This task requires exceptionally strong model capabilities, as it involves generating long-form text with strict formatting constraints (draw.io XML).
+
+**Recommended models**:
+
+-   Claude Sonnet 4.5 / Opus 4.5
+
+**Note on Ollama**: While Ollama is supported as a provider, it's generally not practical for this use case unless you're running high-capability models like DeepSeek R1 or Qwen3-235B locally.
+
+## Recommendations
+
+-   **Best experience**: Use models with vision support (GPT-4o, Claude, Gemini) for image-to-diagram features
+-   **Budget-friendly**: DeepSeek offers competitive pricing
+-   **Privacy**: Use Ollama for fully local, offline operation (requires powerful hardware)
+-   **Flexibility**: OpenRouter provides access to many models through a single API