feat: Finish custom provider implementation

- Add CustomProvidersSection component with provider management
- Add API integration for custom providers
- Implement secure API key storage and management
- Implement UI and settings for custom providers
- Refactor SettingsView and openFile for code quality
- Fix linting issues and improve code organization

Author: Oct4Pie

webview-ui/src/shared/types.ts

@@ -0,0 +1,7 @@
+import type {
+	CustomProviderConfig as ExtCustomProviderConfig,
+	ApiConfiguration as ExtApiConfiguration,
+} from "../../../src/shared/api"
+
+export type CustomProviderConfig = ExtCustomProviderConfig
+export type ApiConfiguration = ExtApiConfiguration