feat(settings): refactor agent auth modes and add model provider authentication

- Split env vars and config file persistence into separate save operations
- Add model_provider_id field to agent_setting for tracking selected provider
- Add "Model Provider" auth mode for Claude Code, Codex CLI, and Gemini CLI
- Add "Custom Endpoint" auth mode for Claude Code (previously only official subscription)
- Unify auth mode labels across all three agents (official subscription / custom endpoint / model provider)
- When model provider is selected, fill api_url and api_key into env and config automatically
- Resolve model provider credentials at ACP connect time as a backend fallback
- Clear provider deletion cascades to agent_setting.model_provider_id
- Claude Code writes API credentials to config.env (ANTHROPIC_BASE_URL / ANTHROPIC_AUTH_TOKEN)
- Codex: switching auth modes patches config.toml instead of clearing it
- Add i18n keys for new auth modes in all 10 supported languages

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src-tauri/src/lib.rs

@@ -347,6 +347,8 @@ mod tauri_app {
                 acp_commands::acp_prepare_npx_agent,
                 acp_commands::acp_uninstall_agent,
                 acp_commands::acp_update_agent_preferences,
+                acp_commands::acp_update_agent_env,
+                acp_commands::acp_update_agent_config,
                 acp_commands::acp_reorder_agents,
                 acp_commands::acp_list_agent_skills,
                 acp_commands::acp_read_agent_skill,