Mechanical migration matching PR #2742's recipe (1/3 of the umbrella
PersistentDialog work tracked in #2679):

- PropDashboardDialog.h now inherits from PersistentDialog; drops the
  manual m_titleBar/m_bodyLayout members and the setFramelessMode decl
  (provided by the base class).
- Constructor delegates to PersistentDialog("HF Propagation Dashboard",
  "PropDashboardDialogGeometry", parent) and installs content into
  bodyWidget() instead of a hand-rolled body widget.
- Drops the standalone setFramelessMode() impl and the inline
  FramelessResizer::install(this) bootstrap.
- MainWindow::showPropDashboard() collapses to a single
  showOrRaisePersistent(m_propDashboardDialog, m_propForecast) call.
- Removed the qobject_cast<PropDashboardDialog*> branch from
  setFramelessWindow(): the m_persistentDialogs registry already
  auto-propagates frameless toggles to every PersistentDialog-derived
  child.
- Retyped m_propDashboardDialog from QPointer<QDialog> to
  QPointer<PropDashboardDialog>.

The user-facing window title is preserved verbatim ("HF Propagation
Dashboard") — Principle III: on-screen labels are inviolable for the
user, so the abbreviation in the recipe was not adopted. closeEvent
recipe step is a no-op for this dialog (no override exists; base class
handles geometry-flush on close).

Geometry persists under AppSettings key PropDashboardDialogGeometry.
No behavior change to refresh logic, charts, panels, or the
NetworkAccessManager fetch path.

Blast radius: risk_score=0.163, 2 high-risk affected (top: MainWindow::MainWindow, MainWindow::buildMenuBar). Both are reached through the usual MainWindow include of PropDashboardDialog.h; all PropDashboardDialog references in src/ are accounted for and updated.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

# Conflicts:
#	src/gui/MainWindow.h