fix(settings): persist custom provider baseUrl on creation (#414)

[YizukiAme]

Summary

Fix custom OpenAI-compatible provider baseUrl not being persisted on creation, causing verify-model and generation requests to fail.

Closes #414

Root Cause

handleAddProvider in components/settings/index.tsx hardcoded baseUrl: '' while storing the user-entered URL in defaultBaseUrl. All downstream consumers (model-selector, provider-config-panel, model-edit-dialog, use-chat-sessions) only read baseUrl, so the server received an empty string and fell back to SDK defaults (e.g. api.openai.com).

Changes

Core fix

• components/settings/utils.ts: New createCustomProviderSettings() that stores the user-entered URL in both baseUrl (for requests) and defaultBaseUrl (for placeholder/preview)
• components/settings/index.tsx: handleAddProvider now uses createCustomProviderSettings()

Migration

• lib/store/settings.ts: promoteLegacyCustomProviderBaseUrls() runs on rehydrate — promotes defaultBaseUrl → baseUrl for existing custom providers (!isBuiltIn && !baseUrl && defaultBaseUrl). Does not affect built-in providers.

Refactor

• components/settings/utils.ts: New createVerifyModelRequest() to unify verify-model request body construction across 3 call sites (model-selector, provider-config-panel, model-edit-dialog)

Tests

• tests/settings/custom-provider-baseurl.test.ts: Tests that createCustomProviderSettings stores baseUrl correctly and createVerifyModelRequest builds the correct request body
• tests/store/settings-server-sync.test.ts: Tests that migration promotes defaultBaseUrl for custom providers and leaves built-in providers untouched

Verification

• npm test: 98 passed (0 failed)
• npx eslint on changed files: all clean

Assisted by Claude Opus 4.6 and GPT 5.4

[wyuc]

LGTM