refactor(startup): improve first-launch import logic with per-table detection

- Replace global `is_empty_for_first_import()` with independent checks:
  - `is_mcp_table_empty()` for MCP server imports
  - `is_prompts_table_empty()` for prompt imports
  - Skills and providers already have built-in idempotency checks

- Fix misleading logs in provider import:
  - Change `import_default_config` return type from `Result<()>` to `Result<bool>`
  - Return `true` when actually imported, `false` when skipped
  - Only log success message when import actually occurred

- Add idempotency protection to `import_from_file_on_first_launch`

This allows each data type to be independently recovered if deleted,
rather than requiring all tables to be empty for any import to trigger.

src-tauri/src/commands/provider.rs

@@ -86,7 +86,7 @@ pub fn switch_provider(
         .map_err(|e| e.to_string())
 }
 
-fn import_default_config_internal(state: &AppState, app_type: AppType) -> Result<(), AppError> {
+fn import_default_config_internal(state: &AppState, app_type: AppType) -> Result<bool, AppError> {
     ProviderService::import_default_config(state, app_type)
 }
 
@@ -94,7 +94,7 @@ fn import_default_config_internal(state: &AppState, app_type: AppType) -> Result
 pub fn import_default_config_test_hook(
     state: &AppState,
     app_type: AppType,
-) -> Result<(), AppError> {
+) -> Result<bool, AppError> {
     import_default_config_internal(state, app_type)
 }
 
@@ -102,9 +102,7 @@ pub fn import_default_config_test_hook(
 #[tauri::command]
 pub fn import_default_config(state: State<'_, AppState>, app: String) -> Result<bool, String> {
     let app_type = AppType::from_str(&app).map_err(|e| e.to_string())?;
-    import_default_config_internal(&state, app_type)
-        .map(|_| true)
-        .map_err(Into::into)
+    import_default_config_internal(&state, app_type).map_err(Into::into)
 }
 
 /// 查询供应商用量