feat: support custom --config path in CLI and GUI

Add config.LoadFrom/SaveTo for arbitrary paths and thread a --config
flag through both entry points:
- CLI: --config loads a file first; explicit flags override file values
- GUI: --config selects the settings file used for load and save
- tests for LoadFrom/SaveTo and default fallback

internal/config/config_test.go

@@ -92,3 +92,33 @@ func TestToFlingerAnnounceDefault(t *testing.T) {
 		t.Error("ToFlinger announce default should be true")
 	}
 }
+
+// TestLoadFromMissing returns defaults for a non-existent custom path.
+func TestLoadFromMissing(t *testing.T) {
+	c, err := LoadFrom(filepath.Join(t.TempDir(), "missing", "config.json"))
+	if err != nil {
+		t.Fatalf("LoadFrom: %v", err)
+	}
+	if c.Port != 9756 {
+		t.Errorf("default port = %d, want 9756", c.Port)
+	}
+}
+
+// TestSaveToLoadFromRoundTrip uses an explicit path independent of pathVar.
+func TestSaveToLoadFromRoundTrip(t *testing.T) {
+	p := filepath.Join(t.TempDir(), "custom", "daemon.json")
+
+	announce := false
+	want := Config{Name: "Daemon", Port: 9901, Quality: 90, FPS: 30, Source: "screen", Audio: true, Announce: &announce}
+	if err := SaveTo(p, want); err != nil {
+		t.Fatalf("SaveTo: %v", err)
+	}
+
+	got, err := LoadFrom(p)
+	if err != nil {
+		t.Fatalf("LoadFrom: %v", err)
+	}
+	if got.Name != want.Name || got.Port != want.Port || *got.Announce != false {
+		t.Errorf("round trip mismatch: got %+v want %+v", got, want)
+	}
+}