Default the update repo to the Gitea host

Settles the first open question in claude.md: the update endpoint is
gitea.apointless.space/bsncubed/ipswap. It was left empty — which disables the
checker — only because the host was unconfirmed.

Load unmarshals over the defaults, so an absent update_repo takes the new
default while an explicitly empty one stays empty. That distinction is the off
switch, and forks depend on it, so it is now pinned by a test.

Adds the config package's first tests while here: the defaulting rules above,
the repair path for a nonsensical mask style or poll interval, and the
guarantee that a corrupt config.json still yields usable defaults rather than
stopping the app from starting.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

internal/config/config_test.go

@@ -0,0 +1,98 @@
+package config
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestLoadMissingFileReturnsDefaults(t *testing.T) {
+	s, err := Load(filepath.Join(t.TempDir(), "config.json"))
+	if err != nil {
+		t.Fatalf("a missing config must not be an error: %v", err)
+	}
+	if s != Default() {
+		t.Errorf("expected defaults, got %+v", s)
+	}
+	if s.UpdateRepo != DefaultUpdateRepo {
+		t.Errorf("update repo default = %q, want %q", s.UpdateRepo, DefaultUpdateRepo)
+	}
+}
+
+// An absent field takes the default, but an explicitly empty one must stay
+// empty — that is how the update check is turned off.
+func TestExplicitEmptyUpdateRepoDisablesTheCheck(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "config.json")
+	if err := os.WriteFile(path, []byte(`{"update_repo": ""}`), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	s, err := Load(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if s.UpdateRepo != "" {
+		t.Errorf("an explicit empty update_repo must be preserved, got %q", s.UpdateRepo)
+	}
+	// The rest of the file was absent, so it should still be defaulted.
+	if s.ActivePollSeconds != Default().ActivePollSeconds {
+		t.Errorf("unset fields should keep their defaults, got %+v", s)
+	}
+}
+
+func TestLoadRepairsNonsense(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "config.json")
+	if err := os.WriteFile(path, []byte(`{"mask_style":"wat","active_poll_seconds":0}`), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	s, err := Load(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if s.MaskStyle != MaskPrefix {
+		t.Errorf("an unknown mask style should fall back to prefix, got %q", s.MaskStyle)
+	}
+	if s.ActivePollSeconds != 30 {
+		t.Errorf("a zero poll interval should be repaired, got %d", s.ActivePollSeconds)
+	}
+}
+
+// A corrupt config must not stop the app from starting.
+func TestLoadCorruptFileReturnsDefaultsAndError(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "config.json")
+	if err := os.WriteFile(path, []byte(`{not json`), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	s, err := Load(path)
+	if err == nil {
+		t.Error("expected an error for a corrupt config")
+	}
+	if s != Default() {
+		t.Errorf("a corrupt config should still yield usable defaults, got %+v", s)
+	}
+}
+
+func TestSaveRoundTrip(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "nested", "config.json")
+
+	want := Default()
+	want.MaskStyle = MaskDotted
+	want.StartWithWindows = true
+	want.UpdateRepo = "https://example.com/o/r"
+	if err := want.Save(path); err != nil {
+		t.Fatal(err)
+	}
+
+	got, err := Load(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got != want {
+		t.Errorf("round trip changed the settings:\n got %+v\nwant %+v", got, want)
+	}
+	if !got.DottedMasks() {
+		t.Error("DottedMasks should be true for MaskDotted")
+	}
+}