fix(desktop): polish Models page — ProvidersPanel

[Astro-Han]

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.

[UncertaintyDeterminesYou4ndMe]

Findings from the #1362/#1363 regression probes, parked here for whoever picks this up after #1421 lands (not claiming while blocked):

1. The Models story bridge is missing a channel — the page throws Cannot read properties of undefined (reading 'isExperimentalEnabled') at every width, so the story baseline is currently broken before any layout audit can start.
2. At the 480px window floor, .providersPanel / section.providerMarket overflow the content column by ~62px (was ~110px before the fix(desktop): polish form-row pages — General and Appearance #1362 .settingsStructuredPage explicit-column fix absorbed part of it). The remaining overflow is inside the panel's own layout, so it will need the same treatment as the other pages: explicit minmax(0,1fr) tracks and wrapping instead of nowrap floors.

All other settings pages now sweep clean 480–1280px, so this page is the last known overflow in the #1303 pass.