Fix RDP/VNC clipboard sync randomly stopping

Local copies were de-duplicated against the last synced text with no
expiry, so one lost send made re-copying the same text a no-op; every
open tab listened to the system clipboard, letting background sessions
overwrite it and forwarding one tab's update to all others; and RDP
never sent the initial Format List MS-RDPECLIP requires after Monitor
Ready, which the server can re-send mid-session (e.g. rdpclip.exe
restart), leaving sync dead until reconnect. Announce failures are now
logged to the event log instead of silently dropped.

Bump version to v2026.9.26.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/session_tab.h

@@ -119,7 +119,11 @@ private:
     SessionConnectOptions m_lastConnectOptions;
     QString m_terminalThemeName;
     int m_terminalFontPointSize;
-    QString m_lastSyncedClipboardText;
+    // Text we just wrote to the system clipboard from a remote update, so
+    // the dataChanged() it triggers isn't echoed straight back out. Only
+    // honored briefly -- see onSystemClipboardChanged().
+    QString m_pendingClipboardEcho;
+    qint64 m_pendingClipboardEchoMs = 0;
     bool m_clipboardSyncSupported;
 
     KodoTerm* m_sshTerminal;