refactor: extract streaming engine into internal/flinger

Move the capture/encode/send pipeline out of cmd/teleportfling into a
GUI-free Engine so the CLI and the desktop app share one implementation:
- flinger: Config/Engine/Status, audio+video+stats loops, pattern source
- config: JSON persistence at ~/.config/teleportfling/config.json
- cmd/teleportfling: thin CLI wrapper around the engine (same flags)
- golangci: extend exclusions to flinger/config

internal/config/config_test.go

@@ -0,0 +1,77 @@
+package config
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+// TestLoadDefaultsWhenMissing verifies Load returns defaults when the file
+// does not exist.
+func TestLoadDefaultsWhenMissing(t *testing.T) {
+	// Point at a non-existent directory so we never touch the real config.
+	old := pathVar
+	pathVar = filepath.Join(t.TempDir(), "nope", "config.json")
+	defer func() { pathVar = old }()
+
+	c, err := Load()
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	if c.Port != 9756 {
+		t.Errorf("default port = %d, want 9756", c.Port)
+	}
+}
+
+// TestSaveLoadRoundTrip writes a config and reads it back.
+func TestSaveLoadRoundTrip(t *testing.T) {
+	old := pathVar
+	pathVar = filepath.Join(t.TempDir(), "config.json")
+	defer func() { pathVar = old }()
+
+	want := Config{
+		Name:        "Studio",
+		Port:        9898,
+		Quality:     95,
+		FPS:         60,
+		Source:      "pattern",
+		Audio:       false,
+		StreamIndex: 1,
+	}
+	if err := Save(want); err != nil {
+		t.Fatalf("Save: %v", err)
+	}
+
+	got, err := Load()
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	if got != want {
+		t.Errorf("round trip mismatch:\n got %+v\nwant %+v", got, want)
+	}
+}
+
+// TestLoadFillsZeroValues ensures a partial file gets defaults filled.
+func TestLoadFillsZeroValues(t *testing.T) {
+	old := pathVar
+	pathVar = filepath.Join(t.TempDir(), "config.json")
+	defer func() { pathVar = old }()
+
+	if err := os.WriteFile(pathVar, []byte(`{"name":"Partial"}`), 0o600); err != nil {
+		t.Fatalf("write: %v", err)
+	}
+
+	c, err := Load()
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	if c.Name != "Partial" {
+		t.Errorf("name = %q, want Partial", c.Name)
+	}
+	if c.Port != 9756 {
+		t.Errorf("port = %d, want filled default 9756", c.Port)
+	}
+	if c.Quality != 80 {
+		t.Errorf("quality = %d, want filled default 80", c.Quality)
+	}
+}