1.2.6

Fix a unity-build collision between the diagnostics and the tunnel probe

The Windows jobs failed to compile: tunnelRunning() and kProbeUrl were defined
in anonymous namespaces in both Diagnostics.cpp and TunnelProbe.cpp, and
TrayMenu.cpp already had a tunnelRunning() of its own. MSVC builds this project
with UNITY_BUILD, which concatenates those files into one translation unit, so
the anonymous namespaces merge and the names redefine each other. macOS does
not use unity builds, so it compiled locally and broke only in CI.

Every file-local symbol these three files introduce now carries a per-file
prefix, which is what the rest of this codebase already does for the same
reason.