Fix upload UI stuck on "Processing files..."

The `uploading` flag was only cleared by `x-init="uploading = false"` on the
file list, which Alpine runs when the element is initialized. Adding a second
batch of files to the same share only patches that existing element, so the
flag stayed true forever: the spinner never went away, the drop zone stayed
disabled and the submit button stayed disabled, making the share impossible
to create.

`updatedFiles()` now dispatches `files-processed`, which runs for every batch,
and the form resets its upload state on that event instead.

Also fixes two adjacent bugs in the drag & drop path: `uploadMultiple()` was
called without callbacks so dropped files showed no progress at all, and
`relativePaths` was replaced instead of appended, shifting every earlier
file's path onto the wrong file on a second drop.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/Livewire/FileUploader.php

@@ -65,8 +65,17 @@ class FileUploader extends Component
         ]);
     }
 
+    /**
+     * Validate a freshly uploaded batch of files.
+     *
+     * Dispatches `files-processed` so the front end can drop its "uploading" state.
+     * This runs for every batch, including additional files added to an existing
+     * selection, which a one-off `x-init` on the file list cannot cover.
+     */
     public function updatedFiles(): void
     {
+        $this->dispatch('files-processed')->self();
+
         $maxFileSize = (int) Setting::get('max_file_size', 100 * 1024 * 1024);
         $maxFileSizeMb = $maxFileSize / (1024 * 1024);
         $maxFilesPerShare = (int) Setting::get('max_files_per_share', 50);