[Feature]: Add option to enter both base url and api key for self-hosted/custom endpoints

[design-suzz]

Problem

Problem: The Add AI credential dialog only had 4 fixed providers — Anthropic, OpenAI, OpenRouter (API key only, no base URL), and Ollama (base URL only, labeled as "Ollama"). There was no option to enter both a base URL AND an API key for a self-hosted/custom endpoint.

Proposed solution

Solution: Added a "Custom endpoint (OpenAI-compatible)" option to the provider dropdown that:

1. Shows both a Base URL field (required) and an API key field (optional)
2. Uses the same OpenAI-compatible backend driver as Ollama under the hood
3. Is wired through all layers — frontend UI, API types, backend handler, model listing, and pricing
4. Also renamed the API key field from "Bearer token (optional)" → "API key (optional)" for clarity

Alternatives considered

No response

Area

None

[DavidBabinec]

There is a PR for this by another user. I haven't tested it yet, feel free to test it and let me know how that worked.

#97