Avoid hidden network traffic graph updates

[johnny9]

What changed

• move network traffic sampling, smoothing, and history retention onto an owned worker thread
• publish immutable traffic snapshots back to the GUI thread only while the Network Traffic page is active
• retain raw samples while hidden and rebuild the requested range when the page becomes active again
• lazy-load the Network Traffic settings page so its graphs do not exist while another settings section is selected
• preserve byte totals as 64-bit values
• rename the native graph gradient property so it no longer shadows QQuickPaintedItem::fillColor
• add C++ and QML coverage for thread affinity, hidden-page suppression, retained history, range changes, page lifecycle, and destruction while active

Why

The previous timer was moved to a background thread, but its timeout updated a GUI-thread model every second regardless of whether the Network Traffic page was visible. That kept smoothing, full-history copies, signal delivery, and graph bindings active in the background. Timer/thread ownership was also not explicit during shutdown.

This keeps the inexpensive raw sample collection running off-thread while avoiding hidden GUI work and making worker shutdown deterministic.

LineGraph also declared its own fillColor property over the property inherited from QQuickPaintedItem, producing a QML property-cache warning. The native property is now named gradientColor; the public NetworkTrafficGraph.fillColor API and rendering behavior are unchanged.

User impact

Opening Network Traffic still shows the samples collected while the page was hidden, and changing the graph range rebuilds the visible history without losing totals. Other settings pages no longer pay for traffic graph updates or retain the graph objects.

Issue

Fixes #611.

Fixes #801.

Graph reset behavior was split from #611 into #806 and is intentionally not included here.

Validation

• full RelWithDebInfo build with app tests enabled
• qmllint -I qml -I test/mocks/qml qml/components/NetworkTrafficGraph.qml qml/pages/node/NetworkTraffic.qml qml/pages/node/NodeSettings.qml test/qml/tst_nodesettings.qml
• bitcoinqml_qmltests: 380 passed, 0 failed
• NetworkTrafficTowerTests: 6 passed, 0 failed
• git diff --check origin/qt6...HEAD

The aggregate unit executable was also run locally. Its Network Traffic suite passed; unrelated existing OptionsModel tests could not write the default Bitcoin data directory inside the managed filesystem sandbox.

[jarolrod]

ACK 35fecc9

Sampling stays on the worker, GUI publish is gated on page activity

Tested manually