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>

config/assets.php

@@ -17,6 +17,7 @@ return [
                 'css/components/app-forms.css',
                 'css/components/app-flash.css',
                 'css/components/app-brand.css',
+                'css/components/app-file-upload.css',
             ],
             'default' => [
                 'css/vendor-overrides/multi-select.css',