Reset heartbeat at startup completion to avoid false watchdog restart

[CJKmkp]

Motivation

• Logs showed the watchdog could detect the main thread as unresponsive immediately after startup because the lastHeartbeat timestamp could be stale at the moment isStartupComplete becomes true, causing a false positive restart.

Description

• In Ink Canvas/App.xaml.cs set lastHeartbeat = startupCompleteHeartbeat inside the mainWindow.Loaded handler right after isStartupComplete = true to align the watchdog baseline with the actual startup-complete moment.

Testing

• Applied the patch and verified the modification appears via repository checks using git diff and git status, confirming the new lastHeartbeat assignment is present (no unit tests were run).

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Codex Task