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.go

@@ -0,0 +1,113 @@
+// Package config loads and saves the teleportfling settings file.
+//
+// The file is JSON at ~/.config/teleportfling/config.json and holds the
+// user-visible knobs exposed by the GUI (name, port, quality, fps, source,
+// audio, monitor). Secrets and runtime state are deliberately excluded.
+package config
+
+import (
+	"encoding/json"
+	"errors"
+	"os"
+	"path/filepath"
+
+	"teleportfling/internal/flinger"
+)
+
+// Config mirrors flinger.Config with JSON tags for persistence.
+type Config struct {
+	Name        string `json:"name"`
+	Port        int    `json:"port"`
+	Quality     int    `json:"quality"`
+	FPS         int    `json:"fps"`
+	Source      string `json:"source"`
+	Audio       bool   `json:"audio"`
+	StreamIndex int    `json:"stream_index"`
+}
+
+// Default returns the default configuration.
+func Default() Config {
+	c := flinger.DefaultConfig()
+	return Config{
+		Name:        c.Name,
+		Port:        c.Port,
+		Quality:     c.Quality,
+		FPS:         c.FPS,
+		Source:      c.Source,
+		Audio:       c.Audio,
+		StreamIndex: c.StreamIndex,
+	}
+}
+
+// ToFlinger converts a persisted config to the engine config.
+func (c Config) ToFlinger() flinger.Config {
+	return flinger.Config{
+		Name:        c.Name,
+		Port:        c.Port,
+		Quality:     c.Quality,
+		FPS:         c.FPS,
+		Source:      c.Source,
+		Audio:       c.Audio,
+		StreamIndex: c.StreamIndex,
+	}
+}
+
+// pathVar is the config file location; overridable in tests.
+var pathVar = func() string {
+	dir, err := os.UserConfigDir()
+	if err != nil {
+		dir = "."
+	}
+	return filepath.Join(dir, "teleportfling", "config.json")
+}()
+
+// Path returns the config file location.
+func Path() string {
+	return pathVar
+}
+
+// Load reads the config file, returning Default when it does not exist.
+func Load() (Config, error) {
+	p := Path()
+	data, err := os.ReadFile(p)
+	if err != nil {
+		if errors.Is(err, os.ErrNotExist) {
+			return Default(), nil
+		}
+		return Config{}, err
+	}
+
+	var c Config
+	if err := json.Unmarshal(data, &c); err != nil {
+		return Config{}, err
+	}
+
+	// Fill any zero values with defaults so a hand-edited file still works.
+	d := Default()
+	if c.Port == 0 {
+		c.Port = d.Port
+	}
+	if c.Quality == 0 {
+		c.Quality = d.Quality
+	}
+	if c.FPS == 0 {
+		c.FPS = d.FPS
+	}
+	if c.Source == "" {
+		c.Source = d.Source
+	}
+	return c, nil
+}
+
+// Save writes the config file, creating the directory if needed.
+func Save(c Config) error {
+	p := Path()
+	if err := os.MkdirAll(filepath.Dir(p), 0o755); err != nil {
+		return err
+	}
+	data, err := json.MarshalIndent(c, "", "  ")
+	if err != nil {
+		return err
+	}
+	return os.WriteFile(p, data, 0o600)
+}