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.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"strconv"
 	"strings"
+	"time"
 
 	"github.com/rivo/tview"
 
@@ -149,3 +150,47 @@ func plural(n int) string {
 	}
 	return "s"
 }
+
+// formatSpeed renders a transfer rate given in KiB/s.
+func formatSpeed(kbPerSec float64) string {
+	switch {
+	case kbPerSec >= 1024:
+		return fmt.Sprintf("%.1f MB/s", kbPerSec/1024)
+	case kbPerSec >= 1:
+		return fmt.Sprintf("%.0f KB/s", kbPerSec)
+	}
+	return "0 KB/s"
+}
+
+// progressBar renders pct (0-100) as a bar of the given width, followed by the percentage.
+func progressBar(pct float64, width int) string {
+	pct = min(max(pct, 0), 100)
+	filled := int(pct / 100 * float64(width))
+	return strings.Repeat("█", filled) + strings.Repeat("░", width-filled) + fmt.Sprintf(" %3.0f%%", pct)
+}
+
+// formatAge renders a unix timestamp relative to now ("5m ago", "3d ago").
+func formatAge(unix int64, now time.Time) string {
+	if unix <= 0 {
+		return ""
+	}
+	d := now.Sub(time.Unix(unix, 0))
+	switch {
+	case d < time.Minute:
+		return "just now"
+	case d < time.Hour:
+		return fmt.Sprintf("%dm ago", int(d.Minutes()))
+	case d < 48*time.Hour:
+		return fmt.Sprintf("%dh ago", int(d.Hours()))
+	}
+	return fmt.Sprintf("%dd ago", int(d.Hours()/24))
+}
+
+// truncate shortens s to at most n runes, ending in "…" if it was cut.
+func truncate(s string, n int) string {
+	r := []rune(s)
+	if len(r) <= n {
+		return s
+	}
+	return string(r[:n-1]) + "…"
+}