* fix(macos): drag the window from split-view pane headers
In the macOS app, split-screen pane header rows (session title strip) now
start a native window drag instead of selecting the title text. The Control
UI posts an openclawWindowDrag script message on mousedown over passive
header chrome; the dashboard window validates the trusted main-frame source
and the in-flight left-mouse press before calling NSWindow.performDrag.
Pane headers are chrome, so they are also no longer text-selectable.
* refactor(macos): move the dashboard failure page out of the window controller
SwiftLint type_body_length: the drag-message handler pushed
DashboardWindowController past 800 lines; the failure-page HTML is
presentation-only and now lives in DashboardFailurePage.
