feat: import model providers from cc-switch

[ijry]

Summary

• add backend preview/import commands for model providers discovered from ~/.cc-switch/cc-switch.db
• add a settings dialog to preview supported providers, show skip reasons, and import selected rows
• add transport types, API wrappers, localized copy, and focused frontend/backend tests

Verification

• pnpm exec vitest run src/components/settings/cc-switch-model-provider-importer.test.tsx src/lib/cc-switch-model-provider-import-types.test.ts
• pnpm exec eslint src/components/settings/cc-switch-model-provider-importer.tsx src/components/settings/cc-switch-model-provider-importer.test.tsx src/components/settings/model-provider-settings.tsx src/lib/api.ts src/lib/types.ts
• pnpm build
• cargo check

Notes

• cargo test cc_switch --lib compiled successfully but the test binary failed to start in this Windows environment with STATUS_ENTRYPOINT_NOT_FOUND.