feat(uploads): HEIC/HEIF support + dynamic format hint on guest upload (#821)

Two of the three things from #821:

- HEIC/HEIF (iPhone) can now be enabled. Sharp's bundled libvips decodes `heif`
  input (verified: sharp.format.heif.input.file === true on 0.34.3 / libvips
  8.17.1), so thumbnails generate. Added heic/heif to EXTENSION_TO_MIME in both
  the backend (uploadSettings.js) and the frontend (fileTypes.ts) maps, which
  are kept in sync. (iOS Safari usually transcodes HEIC→JPEG at file selection,
  but a genuine .heic upload is now handled when it arrives.)

- The upload requirements hint no longer hardcodes "JPEG, PNG or WebP". New
  extensionsToLabel() renders the actually-configured, supported formats (e.g.
  "JPG, PNG, WEBP, MOV"), and upload.fileRequirements interpolates {{formats}}
  across all 8 locales. Unsupported extensions are dropped from the label so it
  never advertises a format the backend would reject.

DNG / camera RAW is deliberately NOT included: Sharp's libvips has no raw loader,
so a DNG would upload then fail thumbnailing (photo → 'failed', no preview).
Proper RAW support (embedded-preview extraction) is a separate PR.

Adds vitest coverage for extensionsToLabel + the HEIC mapping.

frontend/src/i18n/locales/de.json

@@ -159,7 +159,7 @@
     "noCategory": "Keine Kategorie",
     "eventSpecific": "(Veranstaltungsspezifisch)",
     "clickToUpload": "Klicken zum Hochladen oder per Drag & Drop",
-    "fileRequirements": "JPEG, PNG oder WebP (max. {{sizeLimit}}MB pro Datei, {{limit}} Dateien pro Upload)",
+    "fileRequirements": "{{formats}} (max. {{sizeLimit}}MB pro Datei, {{limit}} Dateien pro Upload)",
     "selectedFiles": "Ausgewählte Dateien",
     "uploading": "Wird hochgeladen...",
     "transferring": "Übertragung",

frontend/src/i18n/locales/en.json

@@ -159,7 +159,7 @@
     "noCategory": "No category",
     "eventSpecific": "(Event specific)",
     "clickToUpload": "Click to upload or drag and drop",
-    "fileRequirements": "JPEG, PNG or WebP (max {{sizeLimit}}MB per file, {{limit}} files per upload)",
+    "fileRequirements": "{{formats}} (max {{sizeLimit}}MB per file, {{limit}} files per upload)",
     "selectedFiles": "Selected files",
     "uploading": "Uploading...",
     "transferring": "Transferring",

frontend/src/i18n/locales/es.json

@@ -126,7 +126,7 @@
     "noCategory": "Sin categoría",
     "eventSpecific": "(Específico del evento)",
     "clickToUpload": "Haz clic para subir o arrastra y suelta",
-    "fileRequirements": "JPEG, PNG o WebP (máx. {{sizeLimit}}MB por archivo, {{limit}} archivos por subida)",
+    "fileRequirements": "{{formats}} (máx. {{sizeLimit}}MB por archivo, {{limit}} archivos por subida)",
     "fileRequirementsMedia": "Imágenes JPEG, PNG o WebP y videos MP4/MOV/WEBM (máx. 50MB por archivo, {{limit}} archivos por subida)",
     "unsupportedFiles": "Algunos archivos se omitieron porque el formato no es compatible (usa JPEG/PNG/WebP/MP4/MOV/WEBM).",
     "selectedFiles": "Archivos seleccionados",

frontend/src/i18n/locales/fr.json

@@ -131,7 +131,7 @@
     "noCategory": "Aucune catégorie",
     "eventSpecific": "(Spécifique à l'événement)",
     "clickToUpload": "Cliquez pour téléverser ou glissez-déposez",
-    "fileRequirements": "JPEG, PNG ou WebP (max {{sizeLimit}} Mo par fichier, {{limit}} fichiers par téléversement)",
+    "fileRequirements": "{{formats}} (max {{sizeLimit}} Mo par fichier, {{limit}} fichiers par téléversement)",
     "selectedFiles": "Fichiers sélectionnés",
     "uploading": "Téléversement en cours...",
     "transferring": "Transfert en cours",

frontend/src/i18n/locales/nl.json

@@ -131,7 +131,7 @@
     "noCategory": "Geen categorie",
     "eventSpecific": "(Evenement-specifiek)",
     "clickToUpload": "Klik om te uploaden of sleep bestanden hierheen",
-    "fileRequirements": "JPEG, PNG of WebP (max. {{sizeLimit}} MB per bestand, {{limit}} bestanden per upload)",
+    "fileRequirements": "{{formats}} (max. {{sizeLimit}} MB per bestand, {{limit}} bestanden per upload)",
     "selectedFiles": "Geselecteerde bestanden",
     "uploading": "Uploaden...",
     "uploadComplete": "Upload voltooid!",

frontend/src/i18n/locales/pt.json

@@ -131,7 +131,7 @@
     "noCategory": "Sem categoria",
     "eventSpecific": "(Específico do evento)",
     "clickToUpload": "Clique para enviar ou arraste e solte",
-    "fileRequirements": "JPEG, PNG ou WebP (máx. {{sizeLimit}}MB por arquivo, {{limit}} arquivos por envio)",
+    "fileRequirements": "{{formats}} (máx. {{sizeLimit}}MB por arquivo, {{limit}} arquivos por envio)",
     "selectedFiles": "Arquivos selecionados",
     "uploading": "Enviando...",
     "uploadComplete": "Envio concluído!",

frontend/src/i18n/locales/ru.json

@@ -131,7 +131,7 @@
     "noCategory": "Без категории",
     "eventSpecific": "(Для конкретного события)",
     "clickToUpload": "Нажмите для загрузки или перетащите файлы",
-    "fileRequirements": "JPEG, PNG или WebP (макс. {{sizeLimit}} МБ на файл, {{limit}} файлов за загрузку)",
+    "fileRequirements": "{{formats}} (макс. {{sizeLimit}} МБ на файл, {{limit}} файлов за загрузку)",
     "selectedFiles": "Выбранные файлы",
     "uploading": "Загрузка...",
     "uploadComplete": "Загрузка завершена!",

frontend/src/i18n/locales/sl.json

@@ -131,7 +131,7 @@
     "noCategory": "Brez kategorije",
     "eventSpecific": "(specifično za dogodek)",
     "clickToUpload": "Kliknite za nalaganje ali povlecite in spustite",
-    "fileRequirements": "JPEG, PNG ali WebP (največ {{sizeLimit}} MB na datoteko, {{limit}} datotek na nalaganje)",
+    "fileRequirements": "{{formats}} (največ {{sizeLimit}} MB na datoteko, {{limit}} datotek na nalaganje)",
     "selectedFiles": "Izbrane datoteke",
     "uploading": "Nalaganje...",
     "transferring": "Prenašanje",