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/src/lib/components/document/BulkDocumentEditLayout.svelte

@@ -495,7 +495,7 @@ async function retrySave() {
 					<div
 						role="alert"
 						data-testid="bulk-edit-partial-failure"
-						class="rounded-sm border border-red-300 bg-red-50 px-4 py-3 text-sm text-red-700"
+						class="rounded-sm border border-danger/40 bg-danger/10 px-4 py-3 text-sm text-danger"
 					>
 						<p class="font-medium">
 							{m.bulk_edit_save_partial({
@@ -521,6 +521,7 @@ async function retrySave() {
 				onSave={save}
 				onDiscard={handleDiscard}
 				disabled={saving}
+				editMode={mode === 'edit'}
 			/>
 		</div>
 	</div>