feat: add MiniMax provider presets with global and CN endpoints

[octo-patch]

Reason: The built-in provider preset registry had no MiniMax entry, so users had to type endpoints and model names by hand to reach MiniMax models.

Changes

• Add MiniMax presets to the provider selection registry in react/src/components/settings/AddProviderDialog.tsx. Two presets are provided, one for the global endpoint (https://api.minimax.io/v1/) and one for the China endpoint (https://api.minimaxi.com/v1/), each preloaded with the current MiniMax-M3 and MiniMax-M2.7 text models.
• Register the two presets in PROVIDER_NAME_MAPPING (react/src/constants.ts) with display names and icons so they render with proper labels in the settings and model-selector UI.

This follows the existing pattern for named providers in the repository, which are declared as presets on the frontend and consumed at runtime as custom providers; no backend default or new dependency is required.

Checks

• npx tsc -b (in react/): the two changed files introduce no new type errors; the remaining errors are pre-existing and confined to unrelated files.
• npx eslint src/components/settings/AddProviderDialog.tsx src/constants.ts (in react/): the added entries are lint-clean; the single reported error is a pre-existing any on an unrelated line.