feat(settings): add model provider management with full CRUD support

Add a new settings page for managing API model providers (name, API URL,
API key, applicable agent types). Includes database migration, SeaORM
entity, backend CRUD commands/handlers, frontend settings UI with agent
type filter, add/edit/delete dialogs, and i18n support for all 10 locales.

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

src-tauri/src/models/mod.rs

@@ -2,6 +2,7 @@ pub mod agent;
 pub mod chat_channel;
 pub mod conversation;
 pub mod folder;
+pub mod model_provider;
 pub mod message;
 pub mod system;
 

src-tauri/src/models/model_provider.rs

@@ -0,0 +1,42 @@
+use serde::{Deserialize, Serialize};
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct ModelProviderInfo {
+    pub id: i32,
+    pub name: String,
+    pub api_url: String,
+    pub api_key_masked: String,
+    pub agent_types: Vec<String>,
+    pub created_at: String,
+    pub updated_at: String,
+}
+
+fn mask_api_key(key: &str) -> String {
+    let len = key.len();
+    if len <= 8 {
+        "\u{2022}".repeat(len)
+    } else {
+        format!(
+            "{}{}{}",
+            &key[..4],
+            "\u{2022}".repeat(len.min(20) - 8),
+            &key[len - 4..]
+        )
+    }
+}
+
+impl From<crate::db::entities::model_provider::Model> for ModelProviderInfo {
+    fn from(m: crate::db::entities::model_provider::Model) -> Self {
+        let agent_types: Vec<String> =
+            serde_json::from_str(&m.agent_types_json).unwrap_or_default();
+        Self {
+            id: m.id,
+            name: m.name,
+            api_url: m.api_url,
+            api_key_masked: mask_api_key(&m.api_key),
+            agent_types,
+            created_at: m.created_at.to_rfc3339(),
+            updated_at: m.updated_at.to_rfc3339(),
+        }
+    }
+}