Fix Windows single-instance file handoff
On Windows, opening a model file while OrcaSlicer is already running could
fail when single-instance mode was enabled.
The second launch correctly detected that another instance already existed,
but it could no longer find the running main window to forward the file-open
request. As a result, the new process exited and the file was dropped.
This happened because the existing-instance lookup depended on the native
window title containing "OrcaSlicer". After the titlebar change from #12706,
the title may contain only the project name, so the running instance was no
longer discoverable that way.
Fix this by identifying the existing main window through stable window
properties instead of title text:
- require the expected wxWidgets window class
- require the instance hash properties to exist
- match against the current instance hash
This restores forwarding of externally opened files to the already-running
instance without depending on the current titlebar text.
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