Story 12: Update story and specs for Claude integration

Story Updates:
- Unified model dropdown with section headers (Anthropic, Ollama)
- Auto-detect provider from model name (claude-* prefix)
- API key prompt on first Claude model use
- Secure storage in OS keychain via keyring crate
- 200k token context window for Claude models

Spec Updates (AI_INTEGRATION.md):
- Document Anthropic provider implementation
- Anthropic API protocol (SSE streaming, tool format)
- Tool format conversion between internal and Anthropic formats
- API key storage in OS keychain
- Unified dropdown UI flow

Spec Updates (STACK.md):
- Add keyring crate for secure API key storage
- Add eventsource-stream for Anthropic SSE streaming
- Document automatic provider detection
- Update API key management approach

.living_spec/stories/12_be_able_to_use_claude.md

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+# Story 12: Be Able to Use Claude
+
+## User Story
+As a user, I want to be able to select Claude (via Anthropic API) as my LLM provider so I can use Claude models instead of only local Ollama models.
+
+## Acceptance Criteria
+- [ ] Claude models appear in the unified model dropdown (same dropdown as Ollama models)
+- [ ] Dropdown is organized with section headers: "Anthropic" and "Ollama" with models listed under each
+- [ ] When user first selects a Claude model, a dialog prompts for Anthropic API key
+- [ ] API key is stored securely in OS keychain (macOS Keychain, Windows Credential Manager, Linux Secret Service)
+- [ ] Provider is auto-detected from model name (starts with `claude-` = Anthropic, otherwise = Ollama)
+- [ ] Chat requests route to Anthropic API when Claude model is selected
+- [ ] Streaming responses work with Claude (token-by-token display)
+- [ ] Tool calling works with Claude (using Anthropic's tool format)
+- [ ] Context window calculation accounts for Claude models (200k tokens)
+- [ ] User's model selection persists between sessions
+- [ ] Clear error messages if API key is missing or invalid
+
+## Out of Scope
+- Support for other providers (OpenAI, Google, etc.) - can be added later
+- API key management UI (rotation, multiple keys, view/edit key after initial entry)
+- Cost tracking or usage monitoring
+- Model fine-tuning or custom models
+- Switching models mid-conversation (user can start new session)
+- Fetching available Claude models from API (hardcoded list is fine)
+
+## Technical Notes
+- Anthropic API endpoint: `https://api.anthropic.com/v1/messages`
+- API key should be stored securely (environment variable or secure storage)
+- Claude models support tool use (function calling)
+- Context windows: claude-3-5-sonnet (200k), claude-3-5-haiku (200k)
+- Streaming uses Server-Sent Events (SSE)
+- Tool format differs from OpenAI/Ollama - needs conversion
+
+## Design Considerations
+- Single unified model dropdown with section headers ("Anthropic", "Ollama")
+- Use `<optgroup>` in HTML select for visual grouping
+- API key dialog appears on-demand (first use of Claude model)
+- Store API key in OS keychain using `keyring` crate (cross-platform)
+- Backend auto-detects provider from model name pattern
+- Handle API key in backend only (don't expose to frontend logs)
+- Alphabetical sorting within each provider section
+
+## Implementation Approach
+
+### Backend (Rust)
+1. Add `anthropic` feature/module for Claude API client
+2. Create `AnthropicClient` with streaming support
+3. Convert tool definitions to Anthropic format
+4. Handle Anthropic streaming response format
+5. Add API key storage (encrypted or environment variable)
+
+### Frontend (TypeScript)
+1. Add hardcoded list of Claude models (claude-3-5-sonnet-20241022, claude-3-5-haiku-20241022)
+2. Merge Ollama and Claude models into single dropdown with `<optgroup>` sections
+3. Create API key input dialog/modal component
+4. Trigger API key dialog when Claude model selected and no key stored
+5. Add Tauri command to check if API key exists in keychain
+6. Add Tauri command to set API key in keychain
+7. Update context window calculations for Claude models (200k tokens)
+
+### API Differences
+- Anthropic uses `messages` array format (similar to OpenAI)
+- Tools are called `tools` with different schema
+- Streaming events have different structure
+- Need to map our tool format to Anthropic's format
+
+## Security Considerations
+- API key stored in OS keychain (not in files or environment variables)
+- Use `keyring` crate for cross-platform secure storage
+- Never log API key in console or files
+- Backend validates API key format before making requests
+- Handle API errors gracefully (rate limits, invalid key, network errors)
+- API key only accessible to the app process
+
+## UI Flow
+1. User opens model dropdown → sees "Anthropic" section with Claude models, "Ollama" section with local models
+2. User selects `claude-3-5-sonnet-20241022`
+3. Backend checks keychain for stored API key
+4. If not found → Frontend shows dialog: "Enter your Anthropic API key"
+5. User enters key → Backend stores in OS keychain
+6. Chat proceeds with Anthropic API
+7. Future sessions: API key auto-loaded from keychain (no prompt)