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/db/migration/mod.rs

@@ -8,6 +8,7 @@ mod m20260227_000001_folder_parent_branch;
 mod m20260330_000001_chat_channel;
 mod m20260401_000001_chat_channel_sender_context;
 mod m20260404_000001_model_provider;
+mod m20260406_000001_agent_setting_model_provider;
 pub struct Migrator;
 
 #[async_trait::async_trait]
@@ -22,6 +23,7 @@ impl MigratorTrait for Migrator {
             Box::new(m20260330_000001_chat_channel::Migration),
             Box::new(m20260401_000001_chat_channel_sender_context::Migration),
             Box::new(m20260404_000001_model_provider::Migration),
+            Box::new(m20260406_000001_agent_setting_model_provider::Migration),
         ]
     }
 }