feat(providers): implement secure API key storage and provider management

Add complete provider configuration system with the following features:

- Secure API key storage using Electron's safeStorage encryption
- Provider CRUD operations with IPC handlers
- Lazy-loaded electron-store for ESM compatibility
- Provider settings UI component with add/edit/delete functionality
- API key masking for display (shows first/last 4 chars)
- Basic API key format validation per provider type
- Default provider selection
- Provider enable/disable toggle

New files:
- electron/utils/secure-storage.ts: Encrypted key storage and provider config
- src/stores/providers.ts: Zustand store for provider state
- src/components/settings/ProvidersSettings.tsx: Provider management UI

electron/preload/index.ts

@@ -52,6 +52,17 @@ const electronAPI = {
         'env:setApiKey',
         'env:deleteApiKey',
         // Provider
+        'provider:encryptionAvailable',
+        'provider:list',
+        'provider:get',
+        'provider:save',
+        'provider:delete',
+        'provider:setApiKey',
+        'provider:deleteApiKey',
+        'provider:hasApiKey',
+        'provider:getApiKey',
+        'provider:setDefault',
+        'provider:getDefault',
         'provider:validateKey',
         // Cron
         'cron:list',