Sprint 6: JSON file persistence + real SSH terminal + V1 crypto/knownhosts integration

- go.mod replace directive for git.tukangketik.id/swanadiva/hostkeeper → ../../v1
- store/ package: JSON file persistence for hosts, snippets, keys, settings
  (~/Library/Application Support/hostkeeper/v2/)
- model SSH fields: Hostname, Port, Username, AuthType, Password, KeyID
- sshconn/ package: real SSH client via golang.org/x/crypto/ssh
- Terminal WS: real SSH connection with fallback to mock shell
- Dynamic host list in terminal (server-rendered JSON script tag)
- All mock data defaults removed from model packages
- Settings persist via store/settings.go

app/backend/internal/store/store.go

@@ -0,0 +1,59 @@
+package store
+
+import (
+	"encoding/json"
+	"math/rand"
+	"os"
+	"path/filepath"
+	"sync"
+
+	"git.tukangketik.id/swanadiva/hostkeeper/pkg/crypto"
+)
+
+var dataDir string
+var mu sync.RWMutex
+
+func init() {
+	configDir, err := os.UserConfigDir()
+	if err != nil {
+		configDir = os.TempDir()
+	}
+	dataDir = filepath.Join(configDir, "hostkeeper", "v2")
+	os.MkdirAll(dataDir, 0755)
+}
+
+func readFile(name string, v any) error {
+	path := filepath.Join(dataDir, name)
+	data, err := os.ReadFile(path)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil
+		}
+		return err
+	}
+	if len(data) == 0 {
+		return nil
+	}
+	if crypto.IsEncrypted(string(data)) {
+		return nil
+	}
+	return json.Unmarshal(data, v)
+}
+
+func writeFile(name string, v any) error {
+	path := filepath.Join(dataDir, name)
+	data, err := json.MarshalIndent(v, "", "  ")
+	if err != nil {
+		return err
+	}
+	return os.WriteFile(path, data, 0644)
+}
+
+func randID() string {
+	const letters = "abcdefghijklmnopqrstuvwxyz0123456789"
+	b := make([]byte, 8)
+	for i := range b {
+		b[i] = letters[rand.Intn(len(letters))]
+	}
+	return string(b)
+}