refactor: extract business logic to useProviderActions hook
Major improvements:
- Create `src/hooks/useProviderActions.ts` (147 lines)
- Consolidate provider operations (add, update, delete, switch)
- Extract Claude plugin sync logic
- Extract usage script save logic
- Simplify `App.tsx` (347 → 226 lines, -35%)
- Remove 8 callback functions
- Remove Claude plugin sync logic
- Remove usage script save logic
- Cleaner and more maintainable
- Replace `onNotify` prop with `toast` in:
- `UsageScriptModal.tsx`
- `McpPanel.tsx`
- `McpFormModal.tsx`
- `McpWizardModal.tsx`
- Unified notification system using sonner
Benefits:
- Reduced coupling and improved maintainability
- Business logic isolated in hooks, easier to test
- Consistent notification system across the app
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