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/protocol/commands.go

@@ -110,6 +110,7 @@ const (
 	CommandScreenSetClipboard byte = 25  // COMMAND_SCREEN_SET_CLIPBOARD - set remote clipboard [25][GBK text]
 	CommandNext               byte = 30  // COMMAND_NEXT - "control-side dialog is open, you may stream"
 	CommandShell              byte = 40  // COMMAND_SHELL - ask device to open a shell sub-connection
+	CommandUninstall          byte = 63  // TOKEN_UNINSTALL - uninstall the host agent (single byte)
 	CommandTerminalRsize      byte = 81  // CMD_TERMINAL_RESIZE - [cmd:1][cols:2 LE][rows:2 LE]
 	CmdRestoreConsole         byte = 82  // CMD_RESTORE_CONSOLE - RDP session "归位": switch back to the console session and restart capture
 	CommandScreenKeyframeReq  byte = 169 // COMMAND_SCREEN_KEYFRAME_REQ - force the client to emit an IDR immediately