fix(server): share SSH username validation

Centralize SSH username rules and sanitization so dotted usernames are
accepted consistently across API, onboarding, and Livewire server forms.

tests/Unit/ServerUsernamePatternTest.php

@@ -0,0 +1,19 @@
+<?php
+
+use App\Models\Server;
+use App\Support\ValidationPatterns;
+
+it('provides shared validation rules for SSH usernames', function () {
+    expect(ValidationPatterns::SERVER_USERNAME_PATTERN)->toBe('/^[a-zA-Z0-9._-]+$/');
+    expect(ValidationPatterns::serverUsernameRules())->toContain('regex:'.ValidationPatterns::SERVER_USERNAME_PATTERN);
+
+    expect(preg_match(ValidationPatterns::SERVER_USERNAME_PATTERN, 'deploy.user'))->toBe(1);
+    expect(preg_match(ValidationPatterns::SERVER_USERNAME_PATTERN, 'deploy$user'))->toBe(0);
+});
+
+it('preserves dots when sanitizing server SSH usernames', function () {
+    $server = new Server;
+    $server->user = 'deploy.user';
+
+    expect($server->user)->toBe('deploy.user');
+});