Add MiniMax provider to provider management UI

[octo-patch]

Reason: Add MiniMax as a first-class OpenAI-compatible provider with its default endpoint and current model.

This change adds MiniMax to the provider selector, applies its global endpoint and default model, and treats it as an OpenAI-compatible endpoint for model probing and saved-provider forms. It also adds the MiniMax provider label while preserving the existing provider naming coverage.

Checks:

• zig build test -Dembed-ui=false
• npm run build (in ui)
• git diff --check
• Secret scan

[octo-patch]

Updated the MiniMax presets to include the global and China OpenAI-compatible endpoints and the current MiniMax-M3 and MiniMax-M2.7 model choices, and pushed commit 1ea2fe6. I ran npm --prefix ui run build and bash tests/test_backend.sh; all four CI jobs pass.

[vernonstinebaker]

I'm a little concerned about going down a rathole of adding a bunch of providers one-off that are already supported through the OpenAI-Compatible option. We've already gone down that route with NullClaw but I'm inclined to avoid duplicating that -- which, IMO, was probably a well intentioned but perhaps mistaken approach -- here.

If you have a compelling reason to add MiniMax as a stand alone provider instead of using the OpenAI-Compatible option -- which is exactly the approach I use personally -- argue you're points here and I'll reconsider the PR.

If you don't respond within 15 days I'll presume the proposal to use OpenAI-Compatible is acceptable, and I'll close this PR unmerged.

@DonPrus Can you please weigh in on this?