fix(desktop): polish Models page — ProvidersPanel #1367

Sub-issue of #1303.

Dependency

Blocked by #1421, which changes the same provider connection details, credential reveal/save flow, and Storybook fixtures. Audit the final interaction rather than creating a visual baseline on an in-flight form.

Scope

• Models — ProvidersPanel.tsx
• Product story — Product/Settings/Pages/Models
• Provider stories — Product/Settings/Providers/*

Cover:

• recommended and full provider catalogs, including custom relay entries;
• complete provider configuration and OAuth states;
• post-creation model list and enabled-model management;
• saved, pending, failure, empty, long-name, and narrow states;
• page hierarchy, action density, empty/error treatment, and keyboard/focus behavior.

The Models page must use the shared Settings composition grammar from #1360 where its semantics match. Provider-specific interaction remains local.

Acceptance

• Fix model connection credential reveal and save flow #1421 is merged first.
• The story bridge includes isExperimentalEnabled and every channel required by the audited states.
• Stories cover the full provider form, relay catalog, post-creation model list, empty/error states, hostile long names, and the 480px floor.
• Loading, failure, retry, and empty states follow the shared Settings state grammar.
• No action, model name, credential status, or connection detail is clipped at the supported floor.
• Focus order, keyboard operation, both themes, and one built-app verification pass are complete.