Feature: Web screen wall for multi-device live monitoring

Add a third view to the web UI, the screen wall, which shows several
devices at once in one tab with 1x1 / 2x2 / 3x3 / 4x4 / 5x5 layouts and a
device sidebar on the left. Tiles are monitoring-only; clicking one
enlarges that device into the existing single-screen page for control, and
Back returns to the wall. The same device can be added several times: all
tiles of one device share a single decoder and a single stream.

The wall decodes at most 9 streams on desktop and 4 on touch devices.
Tiles beyond the budget keep receiving frames but have their decoder
closed and show the last frame plus a Paused badge; they are promoted
automatically when a live tile goes away. Subscriptions are reference
counted on the client (streams map with owner sets), so a stream is only
connected on the 0 -> 1 transition and disconnected on 1 -> 0, and the
wall and the single-screen page can share one device without tearing the
stream down.

Server side, WebClient now holds a set of watched devices instead of a
single id. A new multiWatch command flag selects set semantics, while
clients without it keep the old replace-all behaviour, so an old frontend
against the new server is unchanged. connect and disconnect responses echo
id and req so the frontend can route them, input commands carry an id and
are resolved against the watch set (dropped when the set has several
devices and no id), and the per-client watch set is capped at 32.

For fast recovery after a pause or when joining late, the server caches
each device's raw IDR payload and replays it on connect and on a new
keyframe command, then asks the client to force a new IDR. The client gets
COMMAND_SCREEN_KEYFRAME_REQ = 169 and an atomic request flag that is only
cleared after an IDR actually came out of the encoder, so a failed capture
or EAGAIN does not lose the request. The frontend retries the keyframe
request up to three times; an old client that ignores the command simply
recovers on the next natural GOP.

Also fix the keyframe cache double wrapping: BroadcastH264Frame used to
cache the full packet header and HandleConnect wrapped it again. The cache
now stores data + 9, len - 9 and every sender wraps it through
BuildFramePacket, and the cache is cleared on cold start and on resolution
change. StopRemoteDesktop is always called outside m_ClientsMutex, with
the ids collected under the lock first.

Co-Authored-By: deepseek-flash

client/ScreenManager.cpp

@@ -930,6 +930,13 @@ VOID CScreenManager::OnReceive(PBYTE szBuffer, ULONG ulLength)
         SwitchScreen();
         break;
     }
+    case COMMAND_SCREEN_KEYFRAME_REQ: {
+        // Web 屏幕墙恢复/新加入者请求：下一帧强制 IDR，立即重建参考链。
+        if (m_ScreenSpyObject) {
+            m_ScreenSpyObject->RequestKeyFrame();
+        }
+        break;
+    }
     case CMD_FULL_SCREEN: {
         int fullScreen = szBuffer[1];
         iniFile cfg(CLIENT_PATH);