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>

tests/Feature/FileUploadTest.php

@@ -94,6 +94,31 @@ test('file upload with max downloads sets limit', function () {
     expect($share->max_downloads)->toBe(5);
 });
 
+test('every upload batch dispatches files-processed to clear the uploading state', function () {
+    Storage::fake('shares');
+
+    Livewire::test(FileUploader::class)
+        ->set('files', [UploadedFile::fake()->create('first.txt', 64)])
+        ->assertDispatched('files-processed')
+        ->set('files', [UploadedFile::fake()->create('second.txt', 64)])
+        ->assertDispatched('files-processed');
+});
+
+test('files added in multiple batches end up in the same share', function () {
+    Storage::fake('shares');
+
+    Livewire::test(FileUploader::class)
+        ->set('files', [UploadedFile::fake()->create('first.txt', 64)])
+        ->set('files', [UploadedFile::fake()->create('second.txt', 64)])
+        ->call('createShare')
+        ->assertHasNoErrors()
+        ->assertRedirectContains('/share/');
+
+    $share = Share::query()->first();
+
+    expect($share->files->pluck('original_name')->all())->toBe(['first.txt', 'second.txt']);
+});
+
 test('file upload requires at least one file', function () {
     Livewire::test(FileUploader::class)
         ->set('files', [])