## Summary
Adds the ability to edit existing custom AI providers through the
settings UI.

## Changes Made
- **UI Changes:**
  - Added "Edit Provider" button to custom provider dropdown menu
  - Modified `CreateCustomProviderDialog` to support edit mode


- **Backend Changes:**
- Implemented `editCustomLanguageModelProvider` handler in
`language_model_handlers.ts`
  - Added corresponding IPC client method
- Database transaction ensures atomicity when updating provider and
associated models


- **Testing:**
  - Added comprehensive e2e test covering edit functionality
  - Tests verify form pre-population, field updates, and UI persistence
  
  


https://github.com/user-attachments/assets/e8c8600e-4fb7-4816-be95-993ede1224d4



## Closes
Fixes #1232 and #1171
    
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## Summary by cubic
Adds edit support for custom language model providers in Settings. Users
can update provider ID, name, API base URL, and API key env var, with
safe backend updates that also retarget associated models if the ID
changes.

- New Features
  - Added “Edit Provider” option in the custom provider menu.
- Dialog supports edit mode with pre-filled fields, unified loading
state, and update button text.
- New IPC handler to edit providers with validation and a transaction;
updates linked models when IDs change.
- IPC client and preload channel updated; React hook exposes
editProvider mutation with cache invalidation.
  - Added e2e test covering the full edit flow.

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