feat: add Gemini OpenAI compatibility

[LichKing-2234]

Summary

• Add Gemini-only /v1beta/openai compatibility routes for models, chat completions, embeddings, image generations, and videos.
• Preserve OpenAI-compatible multimodal input_audio through the chat conversion path and add Gemini OpenAI-compatible embeddings/images forwarding.
• Accept top-level account export imports and verify the local Gemini OAuth export fixture can import 10 accounts without committing credentials.
• Update frontend Use Key examples to expose Gemini native and OpenAI-compatible base URLs.

Test Plan

• SUB2API_ACCOUNT_IMPORT_FIXTURE=/Users/admin/Downloads/sub2api-account-20260529180922.json go test ./internal/handler/admin -run TestImportDataAcceptsLocalGeminiExportFixture -count=1
• cd backend && go test ./...
• cd frontend && pnpm test:run src/components/keys/__tests__/UseKeyModal.spec.ts
• cd frontend && pnpm typecheck
• git diff --check