Add OAuth-backed CLI providers

[futouyiba]

Summary

Adds OAuth-backed local CLI providers for users who want to reuse existing vendor login flows instead of entering API keys:

• Codex CLI (GPT OAuth) via the official codex login / ChatGPT OAuth flow
• Gemini CLI (Antigravity OAuth) via the official local gemini CLI Google OAuth flow
• explicit OAuth login buttons in the LLM provider settings panel
• a shared Tauri local CLI command layer and frontend transport for CLI-backed providers

This keeps LLM Wiki out of the token handling path: it opens or spawns the official CLIs and does not read or persist OAuth credentials.

Notes

The new Codex/Gemini CLI transports are intentionally non-streaming for the first slice: the app receives the final CLI output and emits it as one assistant response. This keeps the OAuth-backed path simple and avoids depending on private web/IDE tokens.

Validation

• npm run build
• cargo check --manifest-path src-tauri/Cargo.toml
• npm run test:mocks (62 files, 988 tests)