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>

ui/src/lib/api.ts

@@ -24,6 +24,7 @@ import type {
   Settings,
   SettingsUpdate,
   ModelsResponse,
+  ProvidersResponse,
   DevServerStatusResponse,
   DevServerConfig,
   TerminalInfo,
@@ -399,6 +400,10 @@ export async function getAvailableModels(): Promise<ModelsResponse> {
   return fetchJSON('/settings/models')
 }
 
+export async function getAvailableProviders(): Promise<ProvidersResponse> {
+  return fetchJSON('/settings/providers')
+}
+
 export async function getSettings(): Promise<Settings> {
   return fetchJSON('/settings')
 }