feat: allow editing custom providers from model settings

[neversaywanan]

Issue for this PR

Closes #18224

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Adds an edit flow for custom providers from the model settings page.

Custom providers now show an Edit action in Settings > Models, which opens the existing custom provider dialog in edit mode. The form is prefilled from the current provider config, keeps the provider ID locked while editing, and updates the saved config without failing duplicate ID validation for the provider being edited.

When models are removed during editing, they are moved into the provider blacklist so hidden models stay hidden instead of reappearing when the provider is saved again.

How did you verify your code works?

• Ran bun test --preload ./happydom.ts ./src/components/dialog-custom-provider.test.ts from packages/app
• Verified editing an existing provider does not fail duplicate ID validation
• Verified removed models are added to the blacklist during edit saves
• Verified blacklisted models stay hidden when the edit form is seeded
• Manually tested the custom provider flow locally from the model settings page

Screenshots / recordings

TODO: add before/after screenshots or a short recording of editing a custom provider from Settings > Models

2026-03-19.18.04.20.mov

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

Hey! Your PR title Add re-editable custom suppliers. (setting-models-custom suppliers) doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, opencode).

See CONTRIBUTING.md for details.

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

[github-actions]

Thanks for updating your PR! It now meets our contributing guidelines. 👍

[neversaywanan]

@adamdotdevin Hi! CI is blocked with “3 workflows awaiting approval”. Could a maintainer please approve and run the workflows for this PR? Thanks!