feat(settings): empty filename template reverts to recorded original filename (#1071)

Redefine the empty download filename template from a no-op to a bulk
revert: FilenameTemplateUseCase resolves the target from each model's
recorded original_file_name sidecar entry (skipping models without one),
which resolves follow-ups 1 and 2 with a single coherent semantic shared
by the download and bulk-apply paths.

Also replace the browser-native confirm() with a self-managed
confirmation modal (filenameTemplateConfirmModal) that stacks above the
settings modal, since ModalManager would close the settings modal when
opening a registered one.

static/css/components/modal/delete-modal.css

@@ -27,6 +27,12 @@
     justify-content: center;
 }
 
+/* Self-managed by SettingsManager: stacks above the settings modal like the
+   directory picker (settings panels sit at 10000/10002). */
+#filenameTemplateConfirmModal {
+    z-index: 10010;
+}
+
 .delete-modal-content {
     max-width: 500px;
     width: 90%;