fix(buildpack): revert default build pack to nixpacks and reorder selector

Change default build_pack from railpack back to nixpacks in all new
application flows (GithubPrivateRepository, GithubPrivateRepositoryDeployKey,
PublicGitRepository) and reorder the build pack dropdown so Nixpacks
appears before Railpack across all relevant views.

Add feature tests covering the nixpacks default and selector ordering.

tests/Feature/NewApplicationBuildpackDefaultsTest.php

@@ -0,0 +1,43 @@
+<?php
+
+use App\Livewire\Project\New\GithubPrivateRepository;
+use App\Livewire\Project\New\GithubPrivateRepositoryDeployKey;
+use App\Livewire\Project\New\PublicGitRepository;
+use App\Models\Team;
+use App\Models\User;
+use Illuminate\Foundation\Testing\RefreshDatabase;
+use Livewire\Livewire;
+
+uses(RefreshDatabase::class);
+
+beforeEach(function () {
+    $this->team = Team::factory()->create();
+    $this->user = User::factory()->create();
+    $this->team->members()->attach($this->user->id, ['role' => 'owner']);
+
+    $this->actingAs($this->user);
+    session(['currentTeam' => $this->team]);
+});
+
+describe('new application buildpack defaults', function () {
+    test('github app repository flow defaults to nixpacks', function () {
+        Livewire::test(GithubPrivateRepository::class, ['type' => 'private-gh-app'])
+            ->assertSet('build_pack', 'nixpacks');
+    });
+
+    test('deploy key repository flow defaults to nixpacks', function () {
+        Livewire::test(GithubPrivateRepositoryDeployKey::class, ['type' => 'private-deploy-key'])
+            ->assertSet('build_pack', 'nixpacks');
+    });
+
+    test('public repository flow defaults to nixpacks and lists railpack second', function () {
+        Livewire::test(PublicGitRepository::class, ['type' => 'public'])
+            ->assertSet('build_pack', 'nixpacks');
+    });
+
+    test('public repository flow keeps railpack available after branch lookup', function () {
+        Livewire::test(PublicGitRepository::class, ['type' => 'public'])
+            ->set('branchFound', true)
+            ->assertSeeInOrder(['Nixpacks', 'Railpack']);
+    });
+});