Return error from GetDefaultStateDir when home directory unavailable

When os.UserConfigDir() fails, DetermineStateDir falls back to
os.UserHomeDir(). Previously the error from UserHomeDir was discarded,
which could result in a dangerous root-relative path (/.config/...) if
both calls fail.

Now DetermineStateDir returns (string, error) and propagates failures
from both UserConfigDir and UserHomeDir.

Closes #14

internal/secret/helpers_test.go

@@ -0,0 +1,50 @@
+package secret
+
+import (
+	"testing"
+)
+
+func TestDetermineStateDir_ErrorsWhenHomeDirUnavailable(t *testing.T) {
+	// Clear all env vars that could provide a home/config directory.
+	// On Darwin, os.UserHomeDir may still succeed via the password
+	// database, so we also test via an explicit empty-customConfigDir
+	// path to exercise the fallback branch.
+	t.Setenv(EnvStateDir, "")
+	t.Setenv("HOME", "")
+	t.Setenv("XDG_CONFIG_HOME", "")
+
+	result, err := DetermineStateDir("")
+	// On systems where both lookups fail, we must get an error.
+	// On systems where the OS provides a fallback (e.g. macOS pw db),
+	// result should still be valid (non-empty, not root-relative).
+	if err != nil {
+		// Good — the error case is handled.
+		return
+	}
+	if result == "/.config/"+AppID || result == "" {
+		t.Errorf("DetermineStateDir returned dangerous/empty path %q without error", result)
+	}
+}
+
+func TestDetermineStateDir_UsesEnvVar(t *testing.T) {
+	t.Setenv(EnvStateDir, "/custom/state")
+	result, err := DetermineStateDir("")
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if result != "/custom/state" {
+		t.Errorf("expected /custom/state, got %q", result)
+	}
+}
+
+func TestDetermineStateDir_UsesCustomConfigDir(t *testing.T) {
+	t.Setenv(EnvStateDir, "")
+	result, err := DetermineStateDir("/my/config")
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	expected := "/my/config/" + AppID
+	if result != expected {
+		t.Errorf("expected %q, got %q", expected, result)
+	}
+}