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/api/client.go

@@ -0,0 +1,76 @@
+package api
+
+import (
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"strings"
+	"time"
+)
+
+type Client struct {
+	baseURL    string
+	httpClient *http.Client
+	apiKey     string
+}
+
+func NewClient(baseURL, apiKey string) *Client {
+	return &Client{
+		baseURL: strings.TrimRight(baseURL, "/"),
+		apiKey:  apiKey,
+		httpClient: &http.Client{
+			Timeout: 10 * time.Second,
+		},
+	}
+}
+
+func (c *Client) newRequest(method, path string) (*http.Request, error) {
+	req, err := http.NewRequest(method, c.baseURL+path, nil)
+	if err != nil {
+		return nil, err
+	}
+	req.Header.Set("Authorization", "Bearer "+c.apiKey)
+	req.Header.Set("Accept", "application/json")
+	return req, nil
+}
+
+func (c *Client) do(req *http.Request, out any) error {
+	resp, err := c.httpClient.Do(req)
+	if err != nil {
+		return err
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode >= 400 {
+		var e struct {
+			Error string `json:"error"`
+		}
+		_ = json.NewDecoder(resp.Body).Decode(&e)
+		if e.Error != "" {
+			return fmt.Errorf("server returned %d: %s", resp.StatusCode, e.Error)
+		}
+		return fmt.Errorf("server returned %d", resp.StatusCode)
+	}
+
+	if out != nil {
+		return json.NewDecoder(resp.Body).Decode(out)
+	}
+	return nil
+}
+
+// HealthCheck calls GET /healthz (unauthenticated path, no auth needed but we send it anyway).
+func (c *Client) HealthCheck() error {
+	req, err := http.NewRequest(http.MethodGet, c.baseURL+"/healthz", nil)
+	if err != nil {
+		return err
+	}
+	resp, err := c.httpClient.Do(req)
+	if err != nil {
+		return err
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode != http.StatusOK {
+		return fmt.Errorf("health check failed: %s", resp.Status)
+	}
+	return nil
+}