feat(model manager): external providers UI/UX tweaks

[joshistoast]

Summary

Updates the UI for the external providers API keys section in the model manager to be more consistent with the rest of the UI, and addresses some bugs I found.

• Enhance UI design
• Add icons for providers
• Display toasts for updates
• Address some form submission error handling bugs
• Address some api key state bugs

Related Issues / Discussions

QA Instructions

Merge Plan

Checklist

• The PR has a short but descriptive title, suitable for a changelog
• Tests added / updated (if applicable)
• ❗Changes to a redux slice have a corresponding migration
• Documentation added / updated (if applicable)
• Updated What's New copy (if doing a release after this PR)

[Pfannkuchensack]

I like what I see.

[Pfannkuchensack]

Findings

• High: invokeai/app/api/routers/app_info.py:214 ignores explicit base_url: null payloads, breaking the new "Override Base URL" toggle. The updated frontend in invokeai/frontend/web/src/features/modelManagerV2/subpanels/AddModelPanel/ExternalProviders/ExternalProvidersForm.tsx:192-196 posts { base_url: null } when the user disables the override switch on a provider that already has a custom base URL stored. The route handler guards every field with if update.base_url is not None: (line 214) and therefore drops the null value entirely, so:

  1. The persisted external_<provider>_base_url in api_keys.yaml is never removed - the override silently survives.
  2. When the user disables the toggle without also entering a new API key, the resulting updates dict is empty and the handler raises HTTPException(400, "No external provider config fields provided") at line 218-219. The frontend .catch then shows the generic externalProviderSaveFailed toast added at ExternalProvidersForm.tsx:208-213, even though the user input was valid.

  Pydantic accepts base_url: str | None (line 92) and the generated TS type at invokeai/frontend/web/src/services/api/types.ts:57-60 already exposes string | null, so the wire format is the only thing that's right; the route must distinguish "field omitted" from "field explicitly null". Fix candidate: use update.model_fields_set (or check "base_url" in update.model_fields_set) to detect explicit nulls, mirroring the pattern used in UpdateAppGenerationSettingsRequest.validate_explicit_nulls at lines 135-139.

  To expose this issue, add a test that first POSTs {"api_key": "k", "base_url": "https://x"} to /api/v1/app/external_providers/config/openai, then POSTs {"base_url": null} and asserts the response is 200, payload["base_url"] is None, and load_external_api_keys(...) no longer contains external_openai_base_url while still containing external_openai_api_key.

• Low: invokeai/frontend/web/public/locales/en.json:3484 adds the key whatsNew.readDocumentation but no caller references it. The "Read the Docs" link in invokeai/frontend/web/src/features/ui/components/WhatsNew.tsx:37 uses whatsNew.readTheDocs instead. The unused key is dead translation surface and will mislead translators. Remove it or wire it up.