feat(vpn): VPN on-demand Tailscale + agent studioE5 standalone

- Agent studioE5 standalone en Go (console + systray)
- VPN on-demand via tailscaled + tailscale up (authkey Headscale)
- Resolver/serveur dans le tailnet studioe5
- Caddy on-demand TLS pour les instances
- Nouveaux endpoints serveur /api/internal/send-to-node
- Suppression des anciens binaires edubox-agent
- Suivi dans SUIVI_VPN_ONDEMAND.md

agent/config.go

@@ -0,0 +1,68 @@
+package main
+
+import (
+	"encoding/json"
+	"os"
+	"path/filepath"
+)
+
+// AgentConfig holds user-editable settings for the agent.
+type AgentConfig struct {
+	Server           string `json:"server"`
+	HeadscaleURL     string `json:"headscale_url"`
+	HeadscaleAuthKey string `json:"headscale_auth_key"`
+	NodeID           string `json:"node_id"`
+	DataDir          string `json:"data_dir"`
+}
+
+const configFileName = "studioE5-config.json"
+
+// defaultConfig returns sensible defaults for a first run.
+func defaultConfig(dataDir string) *AgentConfig {
+	return &AgentConfig{
+		Server:           "ws://localhost:3001",
+		HeadscaleURL:     "",
+		HeadscaleAuthKey: "",
+		NodeID:           defaultNodeID(),
+		DataDir:          dataDir,
+	}
+}
+
+// configPath returns the absolute path to the config file.
+func configPath(dataDir string) string {
+	return filepath.Join(dataDir, configFileName)
+}
+
+// loadOrCreateConfig loads the config file. If it does not exist, it creates
+// one with default values and returns it (the caller can then open the settings UI).
+func loadOrCreateConfig(dataDir string) (*AgentConfig, bool, error) {
+	cp := configPath(dataDir)
+
+	if _, err := os.Stat(cp); err == nil {
+		data, err := os.ReadFile(cp)
+		if err != nil {
+			return nil, false, err
+		}
+		var cfg AgentConfig
+		if err := json.Unmarshal(data, &cfg); err != nil {
+			return nil, false, err
+		}
+		return &cfg, false, nil
+	}
+
+	cfg := defaultConfig(dataDir)
+	if err := saveConfig(dataDir, cfg); err != nil {
+		return nil, true, err
+	}
+	return cfg, true, nil
+}
+
+// saveConfig writes the config file to disk.
+func saveConfig(dataDir string, cfg *AgentConfig) error {
+	cp := configPath(dataDir)
+	data, err := json.MarshalIndent(cfg, "", "  ")
+	if err != nil {
+		return err
+	}
+	return os.WriteFile(cp, data, 0644)
+}