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/ApplicationGeneralBuildpackSelectorTest.php

@@ -0,0 +1,68 @@
+<?php
+
+use App\Livewire\Project\Application\General;
+use App\Models\Application;
+use App\Models\Environment;
+use App\Models\InstanceSettings;
+use App\Models\PrivateKey;
+use App\Models\Project;
+use App\Models\Server;
+use App\Models\StandaloneDocker;
+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]);
+    InstanceSettings::unguarded(function () {
+        InstanceSettings::updateOrCreate(['id' => 0], []);
+    });
+
+    $this->project = Project::factory()->create(['team_id' => $this->team->id]);
+    $this->environment = Environment::factory()->create(['project_id' => $this->project->id]);
+    $this->privateKey = PrivateKey::create([
+        'name' => 'Test Key',
+        'private_key' => '-----BEGIN OPENSSH PRIVATE KEY-----
+b3BlbnNzaC1rZXktdjEAAAAABG5vbmUAAAAEbm9uZQAAAAAAAAABAAAAMwAAAAtzc2gtZW
+QyNTUxOQAAACBbhpqHhqv6aI67Mj9abM3DVbmcfYhZAhC7ca4d9UCevAAAAJi/QySHv0Mk
+hwAAAAtzc2gtZWQyNTUxOQAAACBbhpqHhqv6aI67Mj9abM3DVbmcfYhZAhC7ca4d9UCevA
+AAAECBQw4jg1WRT2IGHMncCiZhURCts2s24HoDS0thHnnRKVuGmoeGq/pojrsyP1pszcNV
+uZx9iFkCELtxrh31QJ68AAAAEXNhaWxANzZmZjY2ZDJlMmRkAQIDBA==
+-----END OPENSSH PRIVATE KEY-----',
+        'team_id' => $this->team->id,
+    ]);
+    $this->server = Server::factory()->create([
+        'team_id' => $this->team->id,
+        'private_key_id' => $this->privateKey->id,
+    ]);
+    $this->destination = StandaloneDocker::where('server_id', $this->server->id)->first()
+        ?? StandaloneDocker::factory()->create(['server_id' => $this->server->id, 'network' => 'coolify-test']);
+});
+
+test('existing application buildpack selector lists nixpacks before railpack', function () {
+    $application = Application::factory()->create([
+        'environment_id' => $this->environment->id,
+        'destination_id' => $this->destination->id,
+        'destination_type' => StandaloneDocker::class,
+        'build_pack' => 'nixpacks',
+        'static_image' => 'nginx:alpine',
+        'base_directory' => '/',
+        'is_http_basic_auth_enabled' => false,
+        'redirect' => 'no',
+    ]);
+
+    Livewire::test(General::class, ['application' => $application])
+        ->assertSuccessful()
+        ->assertSeeInOrder([
+            '<option value="nixpacks">Nixpacks</option>',
+            '<option value="railpack">Railpack</option>',
+        ], false);
+});