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/models/model_provider.rs

@@ -5,6 +5,7 @@ pub struct ModelProviderInfo {
     pub id: i32,
     pub name: String,
     pub api_url: String,
+    pub api_key: String,
     pub api_key_masked: String,
     pub agent_types: Vec<String>,
     pub created_at: String,
@@ -33,6 +34,7 @@ impl From<crate::db::entities::model_provider::Model> for ModelProviderInfo {
             id: m.id,
             name: m.name,
             api_url: m.api_url,
+            api_key: m.api_key.clone(),
             api_key_masked: mask_api_key(&m.api_key),
             agent_types,
             created_at: m.created_at.to_rfc3339(),