test(ui): add provider settings stories

[Astro-Han]

Summary

• Add desktop Storybook discovery for renderer-owned stories under apps/desktop/stories.
• Add a provider settings storyboard covering loading, load error, empty, configured, problem, detail, add-provider, and OAuth-card states.
• Extend the Storybook baseline contract so PR5 settings coverage remains present and outside src/renderer.

Why

Refs #390 PR5. Provider/OAuth settings are high-risk for polish because they combine async loading, validation, status rollups, details, and account-login state.

Scope

Changed:

• apps/desktop/.storybook/main.ts
• apps/desktop/stories/settings/provider-settings.stories.tsx
• apps/desktop/src/main/__tests__/storybook-baseline-contract.test.ts

Not included:

• No visual polish.
• No Electron main/preload/storage/runtime behavior changes.
• No full SettingsModal Storybook runtime mock.

Verification

• npm run -w @maka/desktop build:main && node --test apps/desktop/dist/main/__tests__/storybook-baseline-contract.test.js
• npm run -w @maka/desktop build-storybook
• npm run typecheck
• node node_modules/typescript/bin/tsc --noEmit --incremental false --target ES2022 --module ESNext --moduleResolution Bundler --strict --skipLibCheck --esModuleInterop --jsx react-jsx --types vite/client apps/desktop/src/global.d.ts apps/desktop/stories/settings/provider-settings.stories.tsx

Fresh-eye review

• Round 1 found one P2: the new story file was not strict-TS clean. Fixed in c27db71d.
• Round 2 found no P0-P3 issues.

User-facing impact

None. Storybook/test-only coverage for reviewability.

Reviewer notes

The story lives in apps/desktop/stories rather than src/renderer so it remains desktop-owned without joining the production renderer typecheck tree.