fix(bulk-edit): pluralization, edit-mode CTA, error UI, real loading state

Elicit C1+C3 — bulk-selection count uses ICU-style plural keys
(bulk_edit_n_selected_one / _other) so n=1 reads as "1 Dokument" instead
of "1 Dokumente". Save CTA in edit mode reads "Anwenden" via the existing
bulk_edit_save_button key; UploadSaveBar grew an editMode prop. Multi-
chunk progress text is now visible (not aria-only).

Felix C2 — bulk-edit page wires the backend error code through
parseBackendError + getErrorMessage instead of falling back to a generic
internal_error.

Felix C5 — editAllMatching no longer swallows fetch failures: the button
shows an inline error with the backend-mapped message (e.g. when the
filter cap is exceeded).

Leonie C8 — replace the literal "…" loading glyph on /documents/bulk-edit
with a spinner + role=status + aria-live=polite + visible "Loading
documents…" text.

Leonie C9 — partial-failure card and bulk-edit page error card now use
the design-system `text-danger` / `bg-danger/10` / `border-danger/40`
tokens (dark-mode safe) instead of raw red palette values.

Leonie C10 + C13 — German plural fixed; EN badges retensed
("+ added" → "+ will be added", "replaced" → "will replace") to match
the future-tense intent of DE/ES.

Refs #225, PR #331

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/messages/de.json

@@ -875,7 +875,8 @@
 	"bulk_title_single": "Neues Dokument",
 	"bulk_title_multi": "Neue Dokumente",
 	"bulk_edit_button": "Massenbearbeitung",
-	"bulk_edit_n_selected": "{count} Dokumente ausgewählt",
+	"bulk_edit_n_selected_one": "1 Dokument ausgewählt",
+	"bulk_edit_n_selected_other": "{count} Dokumente ausgewählt",
 	"bulk_edit_clear_all": "Alles aufheben",
 	"bulk_edit_all_x": "Alle {count} editieren",
 	"bulk_edit_select_document": "Dokument {title} auswählen",
@@ -893,5 +894,7 @@
 	"form_label_archive_folder": "Mappe",
 	"form_helper_archive_folder": "Welche Mappe innerhalb des Kartons?",
 	"bulk_edit_clear_selection": "Auswahl aufheben",
-	"bulk_edit_clear_hint_keyboard": "Esc: Auswahl aufheben"
+	"bulk_edit_clear_hint_keyboard": "Esc: Auswahl aufheben",
+	"bulk_edit_loading": "Dokumente werden geladen…",
+	"bulk_edit_all_x_failed": "Filter konnte nicht abgerufen werden — bitte erneut versuchen."
 }