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/devices.go

@@ -0,0 +1,32 @@
+package store
+
+import "git.tukangketik.id/swanadiva/hostkeeper/v2/internal/model"
+
+type DeviceStore struct {
+	devices []model.Device
+}
+
+func NewDeviceStore() *DeviceStore {
+	s := &DeviceStore{devices: defaultDevices()}
+	mu.RLock()
+	readFile("devices.json", &s.devices)
+	mu.RUnlock()
+	if s.devices == nil {
+		s.devices = defaultDevices()
+	}
+	return s
+}
+
+func (s *DeviceStore) All() []model.Device {
+	mu.RLock()
+	defer mu.RUnlock()
+	return s.devices
+}
+
+func defaultDevices() []model.Device {
+	return []model.Device{
+		{ID: "d1", Name: "MacBook Pro M4", Type: "desktop", OS: "macOS 15 Sequoia", LastSeen: "Just now", Current: true},
+		{ID: "d2", Name: "iPhone 17 Pro", Type: "mobile", OS: "iOS 20", LastSeen: "2 hours ago", Current: false},
+		{ID: "d3", Name: "iPad Air M3", Type: "tablet", OS: "iPadOS 20", LastSeen: "Yesterday", Current: false},
+	}
+}