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/lib/types.ts

@@ -475,6 +475,7 @@ export interface AcpAgentInfo {
   codex_auth_json: string | null
   codex_config_toml: string | null
   cline_secrets_json: string | null
+  model_provider_id: number | null
 }
 
 // Lightweight agent status returned by acp_get_agent_status
@@ -902,6 +903,7 @@ export interface ModelProviderInfo {
   id: number
   name: string
   api_url: string
+  api_key: string
   api_key_masked: string
   agent_types: AgentType[]
   created_at: string