Fix Qt threading violation in MainThreadRunner initialization

[Copilot]

MainThreadRunner is a QObject that was being instantiated in __init__(), which may execute on any thread. Qt objects must be created on the thread where they will live to avoid threading violations.

Changes:

• Moved MainThreadRunner instantiation from __init__() to initGui(), which QGIS guarantees runs on the main thread
• Initialize self.main_runner = None in __init__()
• Added defensive check in run() with warning log if initGui() was not called properly
• Updated stale comment in canvas_refresh.py referencing old initialization location

Before:

def __init__(self, iface):
    self.iface = iface
    self.main_runner = MainThreadRunner()  # Wrong thread

After:

def __init__(self, iface):
    self.iface = iface
    self.main_runner = None  # Deferred

def initGui(self):
    self.main_runner = MainThreadRunner()  # Main thread guaranteed

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