feat(20-windows-client): aplikacja C# WinForms do zdalnego druku etykiet

- System tray app z NotifyIcon + ContextMenuStrip
- Polling API orderPRO (GET /api/print/jobs/pending)
- Pobieranie etykiet PDF i druk przez PdfiumViewer
- Formularz ustawień: URL API, klucz, drukarka, interwał
- Okno logów z historią (ciemny motyw, Consolas)
- Self-contained .NET 8 publish (win-x64)
- Milestone v0.7 Zdalne drukowanie etykiet — COMPLETE

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

clients/windows/OrderPROPrint/Services/PrintApiClient.cs

@@ -0,0 +1,61 @@
+using System.Net.Http.Json;
+using System.Text.Json;
+using OrderPROPrint.Models;
+
+namespace OrderPROPrint.Services;
+
+public class PrintApiClient : IDisposable
+{
+    private readonly HttpClient _httpClient;
+
+    public PrintApiClient(string apiUrl, string apiKey)
+    {
+        _httpClient = new HttpClient
+        {
+            BaseAddress = new Uri(apiUrl.TrimEnd('/') + "/"),
+            Timeout = TimeSpan.FromSeconds(30)
+        };
+        _httpClient.DefaultRequestHeaders.Add("X-Api-Key", apiKey);
+    }
+
+    public async Task<List<PrintJob>> GetPendingJobsAsync()
+    {
+        var response = await _httpClient.GetAsync("api/print/jobs/pending");
+        response.EnsureSuccessStatusCode();
+
+        var json = await response.Content.ReadAsStringAsync();
+        var result = JsonSerializer.Deserialize<PendingJobsResponse>(json);
+        return result?.Jobs ?? new List<PrintJob>();
+    }
+
+    public async Task<byte[]> DownloadLabelAsync(int jobId)
+    {
+        var response = await _httpClient.GetAsync($"api/print/jobs/{jobId}/download");
+        response.EnsureSuccessStatusCode();
+        return await response.Content.ReadAsByteArrayAsync();
+    }
+
+    public async Task<bool> MarkCompleteAsync(int jobId)
+    {
+        var response = await _httpClient.PostAsync($"api/print/jobs/{jobId}/complete", null);
+        return response.IsSuccessStatusCode;
+    }
+
+    public async Task<bool> TestConnectionAsync()
+    {
+        try
+        {
+            var response = await _httpClient.GetAsync("api/print/jobs/pending");
+            return response.IsSuccessStatusCode;
+        }
+        catch
+        {
+            return false;
+        }
+    }
+
+    public void Dispose()
+    {
+        _httpClient.Dispose();
+    }
+}