feat(cli): upgrade command, rich status, and version cache

- Replace `upgrade` stub with real command (alias for `self-update`)
- Also register `update` as alias: `unarr update` works too
- Rewrite `status` to show full config, disk usage, daemon state, and
  update availability with colored sections
- Add version check cache (1h TTL) so `status` is instant on repeat runs
- Guard against division by zero on empty filesystems
- Guard against negative durations from clock skew
- Guard against stale PID via heartbeat recency check (2 min)
- Add comprehensive test coverage across agent, engine, upgrade, usenet,
  arr, library, mediaserver, and UI packages
- Improve Makefile coverage target to exclude cmd/ glue code
- Fix stream handler resource cleanup and ffprobe error handling

internal/cmd/helpers_test.go

@@ -0,0 +1,43 @@
+package cmd
+
+import (
+	"os"
+	"strings"
+	"testing"
+)
+
+func TestExpandHome(t *testing.T) {
+	home, _ := os.UserHomeDir()
+
+	tests := []struct {
+		input string
+		want  string
+	}{
+		{"~/Documents", home + "/Documents"},
+		{"~/", home},
+		{"/absolute/path", "/absolute/path"},
+		{"relative/path", "relative/path"},
+		{"", ""},
+		{"~notexpanded", "~notexpanded"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.input, func(t *testing.T) {
+			got := expandHome(tt.input)
+			if got != tt.want {
+				t.Errorf("expandHome(%q) = %q, want %q", tt.input, got, tt.want)
+			}
+		})
+	}
+}
+
+func TestDefaultDownloadDir(t *testing.T) {
+	dir := defaultDownloadDir()
+	if dir == "" {
+		t.Error("defaultDownloadDir() returned empty string")
+	}
+	home, _ := os.UserHomeDir()
+	if !strings.HasPrefix(dir, home) {
+		t.Errorf("defaultDownloadDir() = %q, expected to start with home dir %q", dir, home)
+	}
+}