feat: add API provider selection UI and fix stuck features on agent crash

API Provider Selection:
- Add provider switcher in Settings modal (Claude, Kimi, GLM, Ollama, Custom)
- Auth tokens stored locally only (registry.db), never returned by API
- get_effective_sdk_env() builds provider-specific env vars for agent subprocess
- All chat sessions (spec, expand, assistant) use provider settings
- Backward compatible: defaults to Claude, env vars still work as override

Fix Stuck Features:
- Add _cleanup_stale_features() to process_manager.py
- Reset in_progress features when agent stops, crashes, or fails healthcheck
- Prevents features from being permanently stuck after rate limit crashes
- Uses separate SQLAlchemy engine to avoid session conflicts with subprocess

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

env_constants.py

@@ -15,6 +15,7 @@ API_ENV_VARS: list[str] = [
     # Core API configuration
     "ANTHROPIC_BASE_URL",              # Custom API endpoint (e.g., https://api.z.ai/api/anthropic)
     "ANTHROPIC_AUTH_TOKEN",            # API authentication token
+    "ANTHROPIC_API_KEY",               # API key (used by Kimi and other providers)
     "API_TIMEOUT_MS",                  # Request timeout in milliseconds
     # Model tier overrides
     "ANTHROPIC_DEFAULT_SONNET_MODEL",  # Model override for Sonnet