Narrow PR scope: drop unintended Qt test/additional UI deltas and keep only required toggle styling behavior

[Copilot]

This updates the branch to the minimal change set needed for the issue. The unintended src/qt/test/platformstyletests.cpp addition (and related wiring) is removed, and unrelated sendcoinsdialog.cpp styling edits are reverted.

• Scope reduction: remove unintended platform style test files

  • Deleted:
    • src/qt/test/platformstyletests.cpp
    • src/qt/test/platformstyletests.h
  • Removed build/test wiring from:
    • src/Makefile.qttest.include
    • src/qt/test/test_main.cpp

• Scope reduction: revert unrelated send coins dialog styling

  • Reverted src/qt/sendcoinsdialog.cpp label stylesheet changes that were not required for this issue.

• Keep required behavior in main-window theme toggle

  • Retained the status-bar dark/light toggle glyph color alignment with PlatformStyle::SingleColor().
  • Applied the style via the existing toggle refresh path so color is refreshed when mode/tint updates.

void BitcoinGUI::updateDarkModeToggleText()
{
    // ...
    themeToggleButton->setStyleSheet(
        QString("QToolButton { color : %1; }").arg(platformStyle->SingleColor().name()));
    // ...
}

void BitcoinGUI::onDarkTintSelected(QAction* action)
{
    // ...
    PlatformStyle::setDarkModeTint(action->data().toInt());
    updateDarkModeToggleText();
}

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