feat(themen): key reader tag visibility on the subtree rollup (#698)

Regenerate the TagTreeNodeDTO type with subtreeDocumentCount and switch
hasAnyDocuments to read it directly — the backend rollup already includes all
descendants, so the recursive children walk is no longer needed. Reader
surfaces now hide a topic only when its whole subtree is empty.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/generated/api.ts

@@ -2230,6 +2230,11 @@ export interface components {
             color?: string;
             /** Format: int32 */
             documentCount: number;
+            /**
+             * Format: int32
+             * @description Distinct documents tagged with this tag or any descendant tag (subtree rollup)
+             */
+            subtreeDocumentCount: number;
             children?: components["schemas"]["TagTreeNodeDTO"][];
             /**
              * Format: uuid