fix(models): preserve empty providers and discovery controls

[Ingwannu]

Summary

Fixes two related model-discovery failures:

• custom Anthropic-compatible providers now fall back to their configured defaultModel when live /models discovery fails and no static models[] exists (新增自定义提供商，如果适配器选用anthropic则看不到模型 #308);
• configured providers no longer disappear from the Models page when they publish zero rows, and liveModels can now be changed from Provider Settings (Feature: Provider model discovery control and empty provider visibility #310).

Root cause

fetchProviderModels() called the failure path fallback=configured, but the configured fallback was built only from models[]; a callable Anthropic defaultModel was discarded. Separately, the Models page derived provider groups exclusively from /api/models, so a zero-row provider had no UI identity. The backend honored liveModels but its field-mask PATCH and normal Settings UI did not expose it.

Changes

• Use an Anthropic-compatible defaultModel only as a failed-live-discovery fallback; successful live catalogs remain authoritative and liveModels:false with an empty static list still publishes zero rows.
• Add strict boolean liveModels PATCH support and non-secret provider discovery metadata.
• Add a Provider Settings live-discovery switch.
• Merge configured provider metadata into Models-page grouping and render explicit zero-model guidance for discovery-on and discovery-off states.
• Exclude disabled and empty forward-only providers from those groups.
• Add six-locale UI strings and dashboard documentation.

Validation

• bun run test: 3,614 passed, 0 failed
• bun run typecheck
• bun run privacy:scan
• bun run lint:gui
• bun run build:gui
• focused API/catalog/grouping/SSR tests

Fixes #308
Fixes #310

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