✨ Surface which files failed during photo upload

A partial upload only told the admin "some files failed" with no way to
find out which ones — even though the data existed. The backend already
returns per-file rejections (response.errors: [{filename, error}]) and the
progress hook already exposes failedPhotos, but both were dropped.

Add a dismissible failure report to the upload modal listing every file
that didn't make it into the gallery, grouped by stage with its reason:
- rejected: per-file validation rejections from the upload response
  (previously discarded entirely)
- transfer: whole-chunk request failures (now captured with the error,
  not just the filename)
- processing: background-worker failures from useUploadProgress.failedPhotos

Replace the count-only "some files failed" toast with one that points at
the list. Add en/de keys under upload.failures.* and a component test
covering the rejected + processing rows and dismissal.

frontend/src/i18n/locales/de.json

@@ -169,6 +169,16 @@
     "retryFailed": "Fehlgeschlagene erneut versuchen",
     "uploadComplete": "Upload abgeschlossen!",
     "someFilesFailed": "Einige Dateien konnten nicht hochgeladen werden",
+    "failures": {
+      "title": "{{count}} Datei(en) konnten nicht hochgeladen werden",
+      "toast": "{{count}} Datei(en) konnten nicht hochgeladen werden – siehe Liste unten.",
+      "kindRejected": "Abgelehnt",
+      "kindTransfer": "Übertragung fehlgeschlagen",
+      "kindProcessing": "Verarbeitung fehlgeschlagen",
+      "transferReason": "Übertragung fehlgeschlagen",
+      "unknownReason": "Unbekannter Fehler",
+      "unknownFile": "Unbekannte Datei"
+    },
     "replaceByName": "Vorhandene Fotos mit gleichem Namen ersetzen",
     "uploadPhotos": "Fotos hochladen",
     "uploadMedia": "Fotos & Videos hochladen",

frontend/src/i18n/locales/en.json

@@ -169,6 +169,16 @@
     "retryFailed": "Retry failed",
     "uploadComplete": "Upload complete!",
     "someFilesFailed": "Some files failed to upload",
+    "failures": {
+      "title": "{{count}} file(s) could not be uploaded",
+      "toast": "{{count}} file(s) could not be uploaded — see the list below.",
+      "kindRejected": "Rejected",
+      "kindTransfer": "Transfer failed",
+      "kindProcessing": "Processing failed",
+      "transferReason": "Transfer failed",
+      "unknownReason": "Unknown error",
+      "unknownFile": "Unknown file"
+    },
     "replaceByName": "Replace existing photos with same name",
     "uploadPhotos": "Upload Photos",
     "uploadMedia": "Upload Photos & Videos",