Add model provider removal controls

[bingqilinweimaotai]

Summary

• add a WebUI mutation for removing model provider configuration
• expose confirmation controls for built-in and custom providers
• prevent removal while model presets or image generation still reference the provider
• show localized, in-dialog feedback when removal is blocked

Why

The WebUI allowed providers to be created and updated, but it did not provide a way to remove saved credentials or delete custom providers. A removal flow also needs to preserve configuration integrity instead of leaving model presets with dangling provider references.

Behavior

• Built-in providers have their saved configuration reset while remaining available in the provider catalog.
• Custom providers are removed from the configuration entirely.
• OAuth providers continue to use their existing sign-out flow.
• Removal is rejected when a default or named model preset explicitly references the provider, including provider-prefixed models resolved through auto.
• Custom providers used by image generation must be replaced there before deletion.
• Validation failures remain visible in the confirmation dialog and are localized across the existing WebUI locales.

No changes were made to the agent loop or runner.

Validation

• uv run --no-sync pytest tests/webui/test_settings_api.py tests/webui/test_settings_routes.py -q — 103 passed
• vitest run src/tests/settings-providers.test.tsx — 10 passed
• uv run --no-sync ruff check nanobot/webui/settings_models.py tests/webui/test_settings_api.py
• tsc -p tsconfig.build.json
• vite build
• git diff --check