feat: add workspace and conversation Tauri commands

Implement list_workspaces and list_conversations commands to enable
frontend access to workspace and conversation data via Tauri IPC.

- Add workspace.rs with list_workspaces command (GET /workspaces)
- Add conversation.rs with list_conversations command (GET /conversations?workspace_id=)
- Register both commands in lib.rs invoke_handler
- Update commands/mod.rs to expose new modules

src-tauri/src/commands/conversation.rs

@@ -0,0 +1,18 @@
+use crate::models::Conversation;
+use crate::state::{AppState, BASE_URL};
+
+#[tauri::command]
+pub async fn list_conversations(
+    state: tauri::State<'_, AppState>,
+    workspace_id: String,
+) -> Result<Vec<Conversation>, String> {
+    state
+        .client
+        .get(format!("{}/conversations?workspace_id={}", BASE_URL, workspace_id))
+        .send()
+        .await
+        .map_err(|e| e.to_string())?
+        .json()
+        .await
+        .map_err(|e| e.to_string())
+}

src-tauri/src/commands/mod.rs

@@ -1 +1,3 @@
+pub mod conversation;
 pub mod user;
+pub mod workspace;

src-tauri/src/commands/workspace.rs

@@ -0,0 +1,15 @@
+use crate::models::Workspace;
+use crate::state::{AppState, BASE_URL};
+
+#[tauri::command]
+pub async fn list_workspaces(state: tauri::State<'_, AppState>) -> Result<Vec<Workspace>, String> {
+    state
+        .client
+        .get(format!("{}/workspaces", BASE_URL))
+        .send()
+        .await
+        .map_err(|e| e.to_string())?
+        .json()
+        .await
+        .map_err(|e| e.to_string())
+}