Fix the security audit's findings

Fixes the nine findings of security audit run 1 on 2.4.1 (T1-T11).

F1: Uploads that are not complete now count only the bytes the server
received, and a chunk that goes above the storage quota gets 422. An
anonymous uploader can no longer fill the quota with declared sizes.

F2: The share password and the admin login have a second limit without
the address (20 attempts per minute per share and per e-mail), because
a forged X-Forwarded-For changes the address.

F3: The system password gate and the admin check now also run on
Livewire updates, so an old uploader snapshot cannot register files.

F4: The session keeps a fingerprint of the system password's hash.
When the admin changes the system password, earlier verifications end.

F5: The page after the upload opens only in the session that created
the share, and only while recipients can still reach the share.

F6: The share password limit counts an attempt before the password is
checked, so parallel attempts cannot pass it.

F7: The system password has the same two limits (5 per address, 20 in
total per minute).

F8: A password change or reset signs the user's other sessions out.

F9: The only admin cannot delete the account, which would open /setup
to every visitor.

The setup wizard also holds the admin password to the password
defaults, and Livewire applies EnsureAdmin to updates.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/Feature/Settings/ProfileUpdateTest.php

@@ -74,3 +74,40 @@ test('correct password must be provided to delete account', function () {
 
     expect($user->fresh())->not->toBeNull();
 });
+
+test('the only admin cannot delete their account', function () {
+    $admin = User::query()->where('is_admin', true)->sole();
+
+    $this->actingAs($admin);
+
+    Livewire::test('pages::settings.delete-user-form')
+        ->set('password', 'password')
+        ->call('deleteUser')
+        ->assertHasErrors(['password'])
+        ->assertNoRedirect();
+
+    expect($admin->fresh())->not->toBeNull();
+    expect(auth()->check())->toBeTrue();
+});
+
+test('an admin can delete their account while another admin remains', function () {
+    $admin = User::factory()->admin()->create();
+
+    $this->actingAs($admin);
+
+    Livewire::test('pages::settings.delete-user-form')
+        ->set('password', 'password')
+        ->call('deleteUser')
+        ->assertHasNoErrors()
+        ->assertRedirect('/');
+
+    expect($admin->fresh())->toBeNull();
+});
+
+test('the only admin\'s account card explains why it cannot be deleted instead of offering the button', function () {
+    $this->actingAs(User::query()->where('is_admin', true)->sole());
+
+    Livewire::test('pages::settings.delete-user-form')
+        ->assertSee('You are the only admin, so this account cannot be deleted.')
+        ->assertDontSee('data-test="delete-user-button"', false);
+});