Run as a single instance; second launch activates the existing window #10

Background

Through 0.7.0, RigCheck could be started any number of times; each launch opened a fresh window. Noticed during the first IC-7300 bench session on 2026-09-21.

Why that is a problem

• Two instances contend for the same COM port. The second one's Run Tests or Find my radio reports "port in use" and blames another program — which is technically true and completely misleading.
• Both instances write %LOCALAPPDATA%\ShackDesk\RigCheck\rigcheck-settings.json; whichever closes last wins, silently discarding the other's changes.
• Two sets of logs and telemetry reports interleave for one session.

There is no use for a second window that a single window does not serve better.

Change

From 0.7.1 RigCheck is single-instance: a second launch brings the existing window to the front and exits. Implemented with a named mutex plus a named event in App.OnStartup; the block is marked in the code with a comment pointing at this issue so the behaviour can be reverted if a reason for multiple instances ever appears (for example driving two radios on two ports at once, which would more likely be done inside one window anyway).

Reverting

Delete the single-instance block at the top of App.OnStartup in App.xaml.cs and the ActivateOnSignal helper below it. Nothing else depends on them.