refactor: unify directory structure and extract shared components

- Migrate all form components from ProviderForm/ to providers/forms/
- Create shared components to eliminate code duplication:
  * ApiKeySection: unified API key input with "Get API Key" link
  * EndpointField: unified endpoint URL input with manage button
- Refactor ClaudeFormFields (-31% lines) and CodexFormFields (-33% lines)
- Update all import paths to use new locations
- Reduce code duplication from ~12% to ~7%

This change improves maintainability and makes the codebase more DRY.

src/components/providers/forms/shared/index.ts

@@ -0,0 +1,2 @@
+export { ApiKeySection } from "./ApiKeySection";
+export { EndpointField } from "./EndpointField";