feat: Stage 1 scaffold — config, API client, placeholder TUI

Sets up the full project structure following the hactl/gokapi-tui
architecture: strict Elm-pattern Bubbletea TUI, YAML config at
~/.config/terdut-tui/config.yaml, REST API client with Bearer auth,
self-update via GitHub Releases, and a three-section tab placeholder
(Alerts / Schedule / Users) that verifies server connectivity on startup.

internal/config/config.go

@@ -0,0 +1,63 @@
+package config
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"time"
+
+	"gopkg.in/yaml.v3"
+)
+
+const defaultRefreshInterval = 30 * time.Second
+
+type Config struct {
+	ServerURL       string
+	APIKey          string
+	RefreshInterval time.Duration
+}
+
+type rawConfig struct {
+	ServerURL       string `yaml:"server_url"`
+	APIKey          string `yaml:"api_key"`
+	RefreshInterval int    `yaml:"refresh_interval,omitempty"` // seconds
+}
+
+func Load() (*Config, error) {
+	dir, err := os.UserConfigDir()
+	if err != nil {
+		return nil, fmt.Errorf("cannot determine config directory: %w", err)
+	}
+
+	path := filepath.Join(dir, "terdut-tui", "config.yaml")
+	data, err := os.ReadFile(path)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil, fmt.Errorf("config file not found at %s\n\nCreate it with:\n  server_url: https://terdut.example.com\n  api_key: <your-api-key>", path)
+		}
+		return nil, fmt.Errorf("cannot read config file: %w", err)
+	}
+
+	var raw rawConfig
+	if err := yaml.Unmarshal(data, &raw); err != nil {
+		return nil, fmt.Errorf("invalid config file: %w", err)
+	}
+
+	if raw.ServerURL == "" {
+		return nil, fmt.Errorf("config: 'server_url' is required")
+	}
+	if raw.APIKey == "" {
+		return nil, fmt.Errorf("config: 'api_key' is required")
+	}
+
+	interval := defaultRefreshInterval
+	if raw.RefreshInterval > 0 {
+		interval = time.Duration(raw.RefreshInterval) * time.Second
+	}
+
+	return &Config{
+		ServerURL:       raw.ServerURL,
+		APIKey:          raw.APIKey,
+		RefreshInterval: interval,
+	}, nil
+}