test(utils): add unit tests for systemd file handling

- Add tests for writing .service and .timer files with `writeUnitFiles`
- Add tests for resolving target paths with `resolveUnitTargetPath`
- Ensure proper file creation, content validation, and path resolution

src/utils/__tests__/fs.test.ts

@@ -0,0 +1,59 @@
+import {
+    assertEquals,
+    assertExists,
+    assertStringIncludes,
+} from 'https://deno.land/std@0.224.0/assert/mod.ts';
+import { join } from 'https://deno.land/std@0.224.0/path/mod.ts';
+import { resolveUnitTargetPath, writeUnitFiles } from '../mod.ts';
+import { TimerOptions } from '../../types/options.ts';
+
+Deno.test('writeUnitFiles schreibt .service und .timer korrekt', async () => {
+    const tmp = await Deno.makeTempDir({ prefix: 'test-units-' });
+
+    const options = {
+        output: tmp,
+        user: false,
+    };
+
+    const name = 'test-backup';
+    const serviceContent = '[Service]\nExecStart=/bin/true';
+    const timerContent = '[Timer]\nOnCalendar=daily';
+
+    const { servicePath, timerPath } = await writeUnitFiles(
+        name,
+        serviceContent,
+        timerContent,
+        options as TimerOptions,
+    );
+
+    // Überprüfe Pfade
+    assertEquals(servicePath, join(tmp, 'test-backup.service'));
+    assertEquals(timerPath, join(tmp, 'test-backup.timer'));
+
+    // Existieren Dateien?
+    assertExists(await Deno.stat(servicePath));
+    assertExists(await Deno.stat(timerPath));
+
+    // Enthält die Datei den erwarteten Inhalt?
+    const readService = await Deno.readTextFile(servicePath);
+    const readTimer = await Deno.readTextFile(timerPath);
+
+    assertStringIncludes(readService, 'ExecStart=/bin/true');
+    assertStringIncludes(readTimer, 'OnCalendar=daily');
+});
+
+Deno.test('resolveUnitTargetPath mit --output', () => {
+    const result = resolveUnitTargetPath({ output: '/tmp/units', user: false });
+    assertEquals(result, '/tmp/units');
+});
+
+Deno.test('resolveUnitTargetPath mit --user ohne output', () => {
+    Deno.env.set('HOME', '/home/maxp');
+    const result = resolveUnitTargetPath({ output: undefined, user: true });
+    assertEquals(result, '/home/maxp/.config/systemd/user');
+});
+
+Deno.test('resolveUnitTargetPath ohne output und ohne user', () => {
+    const result = resolveUnitTargetPath({ output: undefined, user: false });
+    assertEquals(result, '/etc/systemd/system');
+});