feat: add reusable app-file-upload component with drag-and-drop and preview

Replace bare <input type="file"> across all upload locations with a
card-based dropzone component via shared partial. Three visual states:
current server file (thumbnail + Replace/Delete), empty dropzone, and
pending file preview (local FileReader thumbnail + metadata). Delete
actions use data-confirm-message for confirmation dialog.

Centralized as templates/partials/app-file-upload.phtml to prevent
markup drift — documented as deliberate exception to plain-HTML inputs
convention in CLAUDE.md and developer checklist.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/reference-entwickler-checkliste.md

@@ -21,6 +21,7 @@ Kurze Definition of Done vor Merge.
 - [ ] Listen-Filter konsistent: IDs/Enums als Select/MultiSelect statt Freitext
 - [ ] Für persistierte Status/Outcomes nur zentrale Taxonomien aus `lib/Domain/Taxonomy/*` nutzen (keine lokalen String-Literale)
 - [ ] Für neue/angepasste Liste: `filter-schema.php` angelegt/aktualisiert
+- [ ] File-Uploads über `templates/partials/app-file-upload.phtml` (kein inline File-Upload-Markup) — `$fileUpload`-Array mit `name`, `accept`, `hint`, optional `currentSrc`/`deleteAction`
 - [ ] Listen-Titlebar über `templates/partials/app-list-titlebar.phtml` (kein inline `<div class="app-list-titlebar">`)
 - [ ] Listen-Tabs über `templates/partials/app-list-tabs.phtml` (kein inline `<div class="app-list-tabs">`)
 - [ ] Purge-Titlebar-Action über `templates/partials/app-list-purge-action.phtml` (kein inline Purge-Form/Button)