feat(web): refine model availability states and preserve API key preview placeholder (#2226)

* feat(web): clarify model availability and status display

- Rename model availability field from configured to available across backend API and frontend usage

- Keep status as reason classification (configured/unconfigured/unreachable) and show unreachable in UI

- Preserve API key preview even when local service is unreachable

- Update backend tests to assert both availability and status semantics

* fix(web): clarify unreachable model status and wording

- Show unreachable status in model cards instead of API key preview when service is down

- Keep API key placeholder preview in model settings whenever an API key is already saved

- Rename model status wording from configured to available across backend, frontend, and i18n

- Update backend model status tests to match renamed status semantics

* style(web): standardize formatting in handleListModels function

* refactor(web): enforce status field as required to follow backend behavior

web/frontend/src/api/models.ts

@@ -20,7 +20,8 @@ export interface ModelInfo {
   thinking_level?: string
   extra_body?: Record<string, unknown>
   // Meta
-  configured: boolean
+  available: boolean
+  status: "available" | "unconfigured" | "unreachable"
   is_default: boolean
   is_virtual: boolean
 }