Modernise proxy lookup and meassure its duration

[TheOneRing]

26-02-09 14:34:20:555 [ info sync.connectionvalidator ]:	System proxy lookup done NoProxy """:0" ["Tunnel Listen UDP SctpTunnel SctpListen"] for "Test@opencloud..de" duration(0h, 0min, 0s, 251ms)

[Copilot]

Pull request overview

Updates the GUI connection validation flow to modernize system proxy lookup and log how long that lookup takes before proceeding with server/auth checks.

Changes:

• Replaced ClientProxy::lookupSystemProxyAsync(...) + SystemProxyRunnable with a QtConcurrent::run(...) + QFutureWatcher flow in ConnectionValidator::checkServer().
• Removed now-unused proxy-lookup API and helper formatting (printQNetworkProxy) from ClientProxy.
• Added duration logging for system proxy lookup completion.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/gui/connectionvalidator.h	Removes the old systemProxyLookupDone(...) slot declaration.
src/gui/connectionvalidator.cpp	Implements system proxy lookup via QtConcurrent/QFutureWatcher and logs elapsed time.
src/gui/clientproxy.h	Removes async lookup API and SystemProxyRunnable declaration.
src/gui/clientproxy.cpp	Removes async lookup implementation and switches proxy logging away from printQNetworkProxy.

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[dragotin]

LGTM