Fix white native-chrome artifacts on dark thememain

QSS alone doesn't reach everything Qt draws — splitter/dock separator
grip-dot decorations and a checkable QMenu item's tick come from the
app's QPalette, which was never set (stylesheet-only theming), so they
stayed Qt's default light palette under the dark stylesheet:

- MainWindow._apply_theme() now also sets a matching dark QPalette
  (restored to the style's standard palette for light theme).
- QMainWindow::separator and a generic QMenu rule (only #profileMenu was
  styled before — the toolbar's View menu had no objectName and so no
  styling at all) added to style_dark.qss, with QMenu::indicator styled
  as a filled/outline square so checkmarks are visible regardless of
  palette.
- QTabWidget/QTabBar (the bare containers, not ::pane/::tab) had no
  background set, so the strip beside the tab buttons fell back to the
  light palette — fixed globally, covers both the Devices/Channels/Plot
  config tabs and the Settings window since both share the same
  QTabWidget styling.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

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