[designerzhi]

Description

Redesign Settings > Models > Default model as a compact, three-level cascading menu, with an opt-in browser preview for configured providers. This PR is ready for owner review; it does not merge or modify main.

Related Issue

No linked issue; implements the requested Settings UI improvements.

Changes

• Show providers at the first level, up to five curated models at the second, and all remaining models under More models at the third level.
• Open submenus on hover or click; support keyboard navigation and preserve the existing confirmed default-selection flow. Selecting a model closes the entire picker.
• Match the first-level popup width to its trigger, use a 6px gap and compact 4px outer padding, and align header copy with provider labels.
• Keep parent rows highlighted while their submenus are open. Use the checkmark, not a persistent background, to indicate the selected model.
• Reuse the Usage sidebar chevron and add a matching outline check. Keep rounded hover/focus states across provider, model, and More models rows.
• Remove search and visible submenu headings from this Settings picker; the chat picker remains unchanged.
• Add opt-in ?preview=models mock configuration with 11 cloud models, fake credentials, in-memory changes, and no model network requests.
• Update English/Chinese strings, documentation, and regression tests.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change
• Documentation update
• CI/CD or build configuration change
• Refactoring (no functional changes to the shared sidebar chevron)

Testing

• 133 related renderer tests passed across 7 files: browser mock, default picker, Settings model integration, shared groups, chat picker, sidebar, and localization.
• Renderer production build passed (cd desktop/renderer && npm run build), including Vue/TypeScript checking.
• Formatting check and Git whitespace checks passed for all changed source files.
• Focused lint passed for the new/modified selector code. Existing warnings remain in the sidebar and integration-test scaffolding; existing build chunk/annotation warnings also remain.
• Manual browser validation performed in light and dark themes: hover through all levels, persistent parent highlights, neutral selected-model background with outline check, keyboard navigation, popup width/spacing, compact padding, aligned header text, rounded corners, and successful model selection closing all menus.
• Full desktop/main-process test suite (not run; no backend behavior changes).

Preview

Run npm run dev:renderer from desktop, then open:
http://127.0.0.1:5173/?preview=models#/settings/models

Preview credentials are fake, and preview changes reset on reload. Curated models are fixed recommendations, not usage tracking.

Checklist

• Follows existing project patterns and shared icons
• Self-review performed
• Documentation updated
• Regression tests added/updated
• Related existing tests pass