feat: add AppState and domain models

- Introduce state.rs with AppState (HTTP client + token/user_id state)
- Define core models in models.rs (User, Workspace, Conversation, Message, AuthTokenResponse)
- Integrate state management into lib.rs via .manage(AppState::new())

src-tauri/src/models.rs

@@ -0,0 +1,43 @@
+use serde::{Deserialize, Serialize};
+
+#[derive(Serialize, Deserialize, Clone)]
+pub struct User {
+    pub id: String,
+    pub email: String,
+    pub username: String,
+    pub display_name: String,
+    pub avatar_url: Option<String>,
+    pub bio: Option<String>,
+}
+
+#[derive(Serialize, Deserialize)]
+pub struct Workspace {
+    pub id: String,
+    pub name: String,
+    pub slug: String,
+    pub owner_id: String,
+}
+
+#[derive(Serialize, Deserialize)]
+pub struct Conversation {
+    pub id: String,
+    pub workspace_id: Option<String>,
+    pub kind: String,
+    pub name: Option<String>,
+    pub is_public: bool,
+    pub created_by: String,
+}
+
+#[derive(Serialize, Deserialize)]
+pub struct Message {
+    pub id: String,
+    pub conversation_id: String,
+    pub user_id: String,
+    pub content: String,
+    pub created_at: String,
+}
+
+#[derive(Deserialize)]
+pub struct AuthTokenResponse {
+    pub token: String,
+}