FRE-680: Initial project scaffold with auth & API client

- Set up Go module with Cobra CLI skeleton
- Implemented login/logout/session commands with 2FA support
- Created ProtonMail API client with rate limiting
- Added config management for ~/.config/pop/
- Configured CI/CD pipeline with GitHub Actions
- Added Makefile for build/test/lint targets

Files:
- main.go, go.mod, go.sum
- cmd/root.go, cmd/auth.go
- internal/auth/session.go
- internal/config/config.go
- internal/api/client.go
- Makefile, README.md, .gitignore
- .github/workflows/ci.yml

internal/config/config.go

@@ -0,0 +1,78 @@
+package config
+
+import (
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+)
+
+type Config struct {
+	APIBaseURL   string `json:"api_base_url"`
+	TimeoutSec   int    `json:"timeout_sec"`
+	RateLimitReq int    `json:"rate_limit_requests"`
+	RateLimitWin int    `json:"rate_limit_window_sec"`
+}
+
+type ConfigManager struct {
+	configDir string
+	configFile string
+}
+
+func NewConfigManager() *ConfigManager {
+	homeDir, err := os.UserHomeDir()
+	if err != nil {
+		homeDir = "~"
+	}
+
+	configDir := filepath.Join(homeDir, ".config", "pop")
+	return &ConfigManager{
+		configDir:  configDir,
+		configFile: filepath.Join(configDir, "config.json"),
+	}
+}
+
+func (m *ConfigManager) ConfigDir() string {
+	return m.configDir
+}
+
+func (m *ConfigManager) Load() (*Config, error) {
+	defaultConfig := &Config{
+		APIBaseURL:   "https://api.protonmail.ch",
+		TimeoutSec:   30,
+		RateLimitReq: 100,
+		RateLimitWin: 60,
+	}
+
+	data, err := os.ReadFile(m.configFile)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return defaultConfig, nil
+		}
+		return nil, fmt.Errorf("failed to read config: %w", err)
+	}
+
+	var config Config
+	if err := json.Unmarshal(data, &config); err != nil {
+		return nil, fmt.Errorf("failed to parse config: %w", err)
+	}
+
+	return &config, nil
+}
+
+func (m *ConfigManager) Save(config *Config) error {
+	if err := os.MkdirAll(m.configDir, 0755); err != nil {
+		return fmt.Errorf("failed to create config dir: %w", err)
+	}
+
+	data, err := json.MarshalIndent(config, "", "  ")
+	if err != nil {
+		return fmt.Errorf("failed to marshal config: %w", err)
+	}
+
+	if err := os.WriteFile(m.configFile, data, 0600); err != nil {
+		return fmt.Errorf("failed to write config file: %w", err)
+	}
+
+	return nil
+}