Group export feeds by category (#9079)

* Group export feeds by category

## What changed
- Groups feed choices in the import/export page by their existing FreshRSS category.
- Preserves the current export request format and selection behavior.

## Why
The export selector previously mixed feeds from every category, making a category-specific export difficult to assemble.

Closes #5555

* Declare export feed count view data

* Fix select size

---------

Co-authored-by: Gerard Alvear <gerard.alvear@logiqd.me>
Co-authored-by: Alexandre Alapetite <alexandre@alapetite.fr>

app/Controllers/importExportController.php

@@ -32,7 +32,8 @@ class FreshRSS_importExport_Controller extends FreshRSS_ActionController {
 	 * This action displays the main page for import / export system.
 	 */
 	public function indexAction(): void {
-		$this->view->feeds = $this->feedDAO->listFeeds();
+		$this->view->categories = $this->categoryDAO->listCategories();
+		$this->view->feedCount = array_sum(array_map(static fn(FreshRSS_Category $category): int => count($category->feeds()), $this->view->categories));
 		FreshRSS_View::prependTitle(_t('sub.import_export.title') . ' · ');
 		$this->listSqliteArchives();
 	}