Feature: Offline device history with delete and remote uninstall

Persist every device that has ever logged in to the Go server in a small
JSON store (devices.json, overridable with YAMA_DEVICES_FILE and capped by
YAMA_DEVICES_MAX, default 1000 newest). The store follows the users.json
convention: atomic temp-file write with fsync and rename, a 500 ms debounce
for bursts of reconnects, first_seen preserved across logins, and pruning
that evicts offline records before devices that are still connected.
Devices connected at shutdown are stamped offline before the final flush so
last_seen stays accurate across restarts.

The server advertises an offlineDevices capability and device_list now
appends historical devices that are not online, marked with online:false and
last_seen. The web UI shows an Offline checkbox after Total/Showing (state
kept in localStorage), renders offline cards dimmed and dashed, and always
lists them in the screen-wall sidebar. Older servers without the capability
keep the online-only list.

Grid and List cards gain a delete button after the terminal button, admin
only, behind a confirmation dialog. Deleting an offline device removes its
history record; deleting an online device first sends the single-byte
TOKEN_UNINSTALL command (matching the MFC OnOnlineUninstall flow) and then
removes the record too, so the device does not come back as a dead offline
entry. The agent's own disconnect then runs the existing cleanup path:
screen and terminal sub-connections are closed, the hub entry is removed,
and browsers receive device_offline to release their watch sets, wall
sessions, and stream references. Wall mode exposes no delete action.

Tests cover history persistence/pruning/recovery, offline ordering, the
device_list merge, and all three delete outcomes (offline removal, online
uninstall with a single 0x3F byte, and viewer rejection).

Co-Authored-By: deepseek-flash

server/go/hub/hub.go

@@ -165,6 +165,9 @@ type DeviceInfo struct {
 	ActiveWindow string `json:"activeWindow,omitempty"`
 	ConnectedAt  int64  `json:"connected_at"`
 	Online       bool   `json:"online"`
+	// LastSeen is unix seconds of the last login/disconnect. Only populated
+	// for offline entries in device_list; online devices leave it zero.
+	LastSeen int64 `json:"last_seen,omitempty"`
 }
 
 // EventHandler receives notifications about device lifecycle, per-tick live
@@ -240,6 +243,17 @@ func (h *Hub) SetSender(fn SendFunc) {
 	h.sender = fn
 }
 
+// IsOnline reports whether id is currently registered with a live main
+// connection. Callers that need to choose between an online-only command and
+// an offline bookkeeping action should check this first; SendToDevice can
+// still turn into ErrDeviceOffline if the device drops in between.
+func (h *Hub) IsOnline(id string) bool {
+	h.mu.RLock()
+	d, ok := h.devices[id]
+	h.mu.RUnlock()
+	return ok && d.conn != nil
+}
+
 // SendToDevice forwards an already-formed command payload to the device's
 // main connection. data should be the raw command bytes (the sender takes
 // care of framing / compression at the protocol layer).