Add SABnzbd client and Downloads view (M4)

SABnzbd client (queue, history, pause/resume, delete, retry) with lenient
number decoding and API-key-safe errors. Downloads view polls while
visible, shows queue and history, and supports pause, delete (with
confirmation) and retry. Add dev/ with pinned Radarr/Sonarr/SABnzbd
containers wired together for manual testing.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

internal/ui/format_test.go

@@ -3,6 +3,7 @@ package ui
 import (
 	"strings"
 	"testing"
+	"time"
 
 	"git.stinnesbeck.com/nils/arr-tui/internal/arr"
 	"git.stinnesbeck.com/nils/arr-tui/internal/radarr"
@@ -86,3 +87,52 @@ func TestResultKeysAndWithID(t *testing.T) {
 		t.Errorf("series key = %q", r.key())
 	}
 }
+
+func TestFormatSpeed(t *testing.T) {
+	tests := map[float64]string{0: "0 KB/s", 0.4: "0 KB/s", 512: "512 KB/s", 1536: "1.5 MB/s"}
+	for in, want := range tests {
+		if got := formatSpeed(in); got != want {
+			t.Errorf("formatSpeed(%v) = %q, want %q", in, got, want)
+		}
+	}
+}
+
+func TestProgressBar(t *testing.T) {
+	if got := progressBar(50, 10); got != "█████░░░░░  50%" {
+		t.Errorf("got %q", got)
+	}
+	if got := progressBar(150, 4); got != "████ 100%" {
+		t.Errorf("clamp high: %q", got)
+	}
+	if got := progressBar(-5, 4); got != "░░░░   0%" {
+		t.Errorf("clamp low: %q", got)
+	}
+}
+
+func TestFormatAge(t *testing.T) {
+	now := time.Unix(1_000_000, 0)
+	tests := map[int64]string{
+		0:                      "",
+		now.Unix() - 10:        "just now",
+		now.Unix() - 5*60:      "5m ago",
+		now.Unix() - 3*3600:    "3h ago",
+		now.Unix() - 3*24*3600: "3d ago",
+	}
+	for in, want := range tests {
+		if got := formatAge(in, now); got != want {
+			t.Errorf("formatAge(%d) = %q, want %q", in, got, want)
+		}
+	}
+}
+
+func TestTruncate(t *testing.T) {
+	if got := truncate("short", 10); got != "short" {
+		t.Errorf("got %q", got)
+	}
+	if got := truncate("Aborted, cannot be completed", 10); got != "Aborted, …" {
+		t.Errorf("got %q", got)
+	}
+	if got := truncate("äöüäöü", 4); got != "äöü…" {
+		t.Errorf("rune handling: %q", got)
+	}
+}