fix: carry providerKind through IPC JSON instead of matching user copy

[KazeKaze93]

Summary

• throwProviderSearchIpcError JSON-encodes the full ProviderSearchErrorPayload (including providerKind) into Error.message, so kind survives Electron invoke which drops custom Error fields.
• Renderer parse no longer infers kind by matching PROVIDER_SEARCH_USER_MESSAGES text. User copy is provider-agnostic (no hardcoded Rule34).
• BrowseErrorState kind values and use sites unchanged.

Test plan

• Gelbooru network error — BrowseErrorState icon/text, no Rule34
• Rule34 network error — no regression
• Rate limit on both providers — kind still rate_limit
• npm run typecheck / lint / test — already green (46 files / 314 tests)